diff --git a/app/routes/webhooks.orders.create.tsx b/app/routes/webhooks.orders.create.tsx
index 0450b7c..8598d0c 100644
--- a/app/routes/webhooks.orders.create.tsx
+++ b/app/routes/webhooks.orders.create.tsx
@@ -5,6 +5,8 @@ import {
   generateAndEmailInvoice,
   isManualPaymentOrder,
 } from "../services/invoice/automations.server";
+import { isDuplicateWebhook } from "../services/webhooks/dedupe.server";
+import { runWebhookInBackground } from "../services/webhooks/background.server";
 
 /**
  * orders/create — Automation 1: when a wire-transfer (manual-payment-gateway)
@@ -16,31 +18,42 @@ export const action = async ({ request }: ActionFunctionArgs) => {
   const { shop, topic, payload, session, admin } = await authenticate.webhook(request);
   console.log(`Received ${topic} webhook for ${shop}`);
 
-  if (!session || !admin) return new Response();
+  // Drop Shopify retries we've already processed (prevents the duplicate
+  // invoice email we saw when the first delivery exceeded Shopify's 5s ack
+  // timeout — the work still completed, but Shopify resent the webhook).
+  if (await isDuplicateWebhook(request, shop, topic)) return new Response();
 
-  const settings = await db.shopSettings.findUnique({ where: { shopDomain: shop } });
-  if (!settings?.autoEmailOnWireTransferPlaced) return new Response();
+  if (!session || !admin) return new Response();
 
   const orderId = payload?.id;
   if (orderId == null) return new Response();
 
-  const orderGid = `gid://shopify/Order/${String(orderId).replace(/^.*\//, "")}`;
-  const isManual = await isManualPaymentOrder(admin, orderGid);
-  if (!isManual) return new Response();
+  const customerLocale =
+    typeof payload?.customer_locale === "string" ? payload.customer_locale : undefined;
+
+  // Respond 200 immediately and run the (slow) PDF + email work in the
+  // background — keeps us well under Shopify's ~5s ack timeout. Dedupe
+  // above guarantees we don't double-process a retry while the first
+  // delivery is still working.
+  runWebhookInBackground(`${topic} order=${orderId} shop=${shop}`, async () => {
+    const settings = await db.shopSettings.findUnique({ where: { shopDomain: shop } });
+    if (!settings?.autoEmailOnWireTransferPlaced) return;
+
+    const orderGid = `gid://shopify/Order/${String(orderId).replace(/^.*\//, "")}`;
+    if (!(await isManualPaymentOrder(admin, orderGid))) return;
 
-  try {
     const result = await generateAndEmailInvoice({
       shopDomain: shop,
       admin,
       orderId,
-      customerLocale: typeof payload?.customer_locale === "string" ? payload.customer_locale : undefined,
+      customerLocale,
     });
     if (!result.ok) {
-      console.warn(`auto-email (wire-transfer placed) failed for order ${orderId} on ${shop}: ${result.reason}`);
+      console.warn(
+        `auto-email (wire-transfer placed) failed for order ${orderId} on ${shop}: ${result.reason}`,
+      );
     }
-  } catch (err) {
-    console.error(`auto-email (wire-transfer placed) crashed for order ${orderId} on ${shop}:`, err);
-  }
+  });
 
   return new Response();
 };
diff --git a/app/routes/webhooks.orders.fulfilled.tsx b/app/routes/webhooks.orders.fulfilled.tsx
index 807bcaa..201ca2d 100644
--- a/app/routes/webhooks.orders.fulfilled.tsx
+++ b/app/routes/webhooks.orders.fulfilled.tsx
@@ -5,6 +5,8 @@ import {
   generateAndEmailInvoice,
   isManualPaymentOrder,
 } from "../services/invoice/automations.server";
+import { isDuplicateWebhook } from "../services/webhooks/dedupe.server";
+import { runWebhookInBackground } from "../services/webhooks/background.server";
 
 /**
  * orders/fulfilled — Automation 2: when an order is fulfilled and is NOT a
@@ -16,36 +18,42 @@ export const action = async ({ request }: ActionFunctionArgs) => {
   const { shop, topic, payload, session, admin } = await authenticate.webhook(request);
   console.log(`Received ${topic} webhook for ${shop}`);
 
+  // Idempotency against Shopify retries — see webhooks/dedupe.server.ts.
+  if (await isDuplicateWebhook(request, shop, topic)) return new Response();
+
   if (!session || !admin) {
     // App was uninstalled before the webhook drained — nothing to do.
     return new Response();
   }
 
-  const settings = await db.shopSettings.findUnique({ where: { shopDomain: shop } });
-  if (!settings?.autoEmailOnFulfilledNonWireTransfer) return new Response();
-
   const orderId = payload?.id;
   if (orderId == null) return new Response();
 
-  const orderGid = `gid://shopify/Order/${String(orderId).replace(/^.*\//, "")}`;
-  if (await isManualPaymentOrder(admin, orderGid)) {
-    // Manual / wire-transfer order — handled by Automation 1, skip here.
-    return new Response();
-  }
+  const customerLocale =
+    typeof payload?.customer_locale === "string" ? payload.customer_locale : undefined;
+
+  // Respond fast; do the heavy lifting after the response (see notes in
+  // webhooks.orders.create.tsx for the rationale).
+  runWebhookInBackground(`${topic} order=${orderId} shop=${shop}`, async () => {
+    const settings = await db.shopSettings.findUnique({ where: { shopDomain: shop } });
+    if (!settings?.autoEmailOnFulfilledNonWireTransfer) return;
+
+    const orderGid = `gid://shopify/Order/${String(orderId).replace(/^.*\//, "")}`;
+    if (await isManualPaymentOrder(admin, orderGid)) {
+      // Manual / wire-transfer order — handled by Automation 1, skip here.
+      return;
+    }
 
-  try {
     const result = await generateAndEmailInvoice({
       shopDomain: shop,
       admin,
       orderId,
-      customerLocale: typeof payload?.customer_locale === "string" ? payload.customer_locale : undefined,
+      customerLocale,
     });
     if (!result.ok) {
       console.warn(`auto-email (fulfilled) failed for order ${orderId} on ${shop}: ${result.reason}`);
     }
-  } catch (err) {
-    console.error(`auto-email (fulfilled) crashed for order ${orderId} on ${shop}:`, err);
-  }
+  });
 
   return new Response();
 };
diff --git a/app/routes/webhooks.orders.updated.tsx b/app/routes/webhooks.orders.updated.tsx
index 0039d3a..1c47386 100644
--- a/app/routes/webhooks.orders.updated.tsx
+++ b/app/routes/webhooks.orders.updated.tsx
@@ -1,10 +1,13 @@
 import type { ActionFunctionArgs } from "react-router";
 import { authenticate } from "../shopify.server";
+import { isDuplicateWebhook } from "../services/webhooks/dedupe.server";
 
 // Acknowledged but not yet acted on. Future: invalidate cached invoice
 // snapshots when a relevant field on the order changes.
 export const action = async ({ request }: ActionFunctionArgs) => {
   const { shop, topic } = await authenticate.webhook(request);
   console.log(`Received ${topic} webhook for ${shop}`);
+  // Idempotency against Shopify retries — see webhooks/dedupe.server.ts.
+  if (await isDuplicateWebhook(request, shop, topic)) return new Response();
   return new Response();
 };
diff --git a/app/services/invoice/i18n.ts b/app/services/invoice/i18n.ts
index 0781f60..d898b0a 100644
--- a/app/services/invoice/i18n.ts
+++ b/app/services/invoice/i18n.ts
@@ -167,11 +167,19 @@ const en: InvoiceStrings = {
   paidStamp: "PAID",
 };
 
+// Locale → invoice language. We only render in German (`de`) when the
+// caller is explicitly German-speaking (de, de-AT, de-DE, de_CH, …).
+// Everything else (it, fr, es, en, …) falls back to English so that
+// non-German-speaking customers don't receive a German invoice. Callers
+// that have a per-shop default fall back to it via
+// `pickLanguage(customerLocale ?? settings.defaultLanguage)`, which is why
+// `null`/`undefined` still maps to `de` (the legacy default for the
+// Austrian shops this app was built for).
 export function pickLanguage(input: string | null | undefined): InvoiceLanguage {
   if (!input) return "de";
   const v = input.toLowerCase();
-  if (v.startsWith("en")) return "en";
-  return "de";
+  if (v.startsWith("de")) return "de";
+  return "en";
 }
 
 export function getStrings(language: InvoiceLanguage): InvoiceStrings {
diff --git a/app/services/webhooks/background.server.ts b/app/services/webhooks/background.server.ts
new file mode 100644
index 0000000..37c77d7
--- /dev/null
+++ b/app/services/webhooks/background.server.ts
@@ -0,0 +1,27 @@
+/**
+ * Fire-and-forget runner for webhook side-effects.
+ *
+ * Shopify expects a 200 response within ~5 seconds, otherwise it considers
+ * the delivery failed and retries it. Heavy automation work (PDF render,
+ * Shopify Files upload, SMTP send) routinely exceeded that budget, which
+ * caused duplicate invoice emails before we added the dedupe table.
+ *
+ * Returning the response immediately and letting the work finish in the
+ * background keeps Shopify happy. Combined with the dedupe table this is
+ * defence-in-depth: dedupe ensures *correctness* even if a retry sneaks
+ * through, while async processing makes retries unlikely in the first
+ * place.
+ *
+ * Errors are caught and logged \u2014 they cannot reach a dispatcher because
+ * the HTTP response is already gone.
+ */
+export function runWebhookInBackground(
+  description: string,
+  work: () => Promise<unknown>,
+): void {
+  // `void` so we don't accidentally `await` the floating promise; the
+  // node event loop keeps the task alive until it settles.
+  void work().catch((err) => {
+    console.error(`background webhook task '${description}' failed:`, err);
+  });
+}
diff --git a/app/services/webhooks/dedupe.server.ts b/app/services/webhooks/dedupe.server.ts
new file mode 100644
index 0000000..272e07c
--- /dev/null
+++ b/app/services/webhooks/dedupe.server.ts
@@ -0,0 +1,67 @@
+import db from "../../db.server";
+
+/**
+ * Minimal shape of the Prisma client surface we use — declared inline so
+ * the helper can be unit-tested with a tiny stub instead of pulling in a
+ * real database.
+ */
+export interface DedupeDeps {
+  db: {
+    processedWebhook: {
+      create: (args: {
+        data: { webhookId: string; topic: string; shopDomain: string };
+      }) => Promise<unknown>;
+    };
+  };
+}
+
+/**
+ * Returns `true` when this Shopify webhook delivery has already been
+ * processed and the caller should short-circuit without doing the work.
+ *
+ * Shopify retries webhook deliveries when it doesn't receive a 200 within
+ * its (~5s) timeout window. Without dedupe this caused us to email an
+ * invoice twice for the same order: the first slow delivery completed its
+ * work but Shopify timed out and re-sent the webhook, which then ran the
+ * automation a second time.
+ *
+ * We key on the `X-Shopify-Webhook-Id` header — Shopify guarantees the same
+ * value for retries of the same delivery, but a new value for genuinely
+ * new events. The insert is the lock: a unique-constraint violation
+ * (Prisma error code `P2002`) means another delivery already claimed this
+ * id.
+ */
+export async function isDuplicateWebhook(
+  request: Request,
+  shop: string,
+  topic: string,
+  deps: DedupeDeps = { db },
+): Promise<boolean> {
+  const webhookId = request.headers.get("x-shopify-webhook-id");
+  if (!webhookId) {
+    // Defensive: in unit tests / non-Shopify callers there is no id.
+    // Don't dedupe — that would silently drop legitimate calls.
+    return false;
+  }
+  try {
+    await deps.db.processedWebhook.create({
+      data: { webhookId, topic, shopDomain: shop },
+    });
+    return false;
+  } catch (err) {
+    // Duck-typed P2002 check so callers can stub the db without pulling
+    // in the real `Prisma` namespace.
+    if ((err as { code?: string } | null)?.code === "P2002") {
+      console.log(
+        `dedupe: skipping duplicate ${topic} delivery for ${shop} (webhookId=${webhookId})`,
+      );
+      return true;
+    }
+    // Don't fail the webhook on a logging-table issue; just process it.
+    console.warn(
+      `dedupe: failed to record webhook ${webhookId} (${topic}/${shop}):`,
+      err,
+    );
+    return false;
+  }
+}
diff --git a/package-lock.json b/package-lock.json
index 4634db4..3282f47 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12,6 +12,7 @@
         "@prisma/client": "^6.16.3",
         "@react-pdf/renderer": "^4.5.1",
         "@react-router/dev": "^7.12.0",
+        "@react-router/express": "^7.14.2",
         "@react-router/fs-routes": "^7.12.0",
         "@react-router/node": "^7.12.0",
         "@react-router/serve": "^7.12.0",
@@ -26,7 +27,10 @@
         "@tiptap/react": "^3.23.1",
         "@tiptap/starter-kit": "^3.23.1",
         "@types/nodemailer": "^8.0.0",
+        "compression": "^1.8.1",
+        "express": "^4.22.1",
         "isbot": "^5.1.31",
+        "morgan": "^1.10.1",
         "nodemailer": "^8.0.7",
         "prisma": "^6.16.3",
         "qrcode": "^1.5.4",
@@ -61,6 +65,15 @@
         "node": ">=20.19 <22 || >=22.12"
       }
     },
+    "extensions/customer-account-payment": {
+      "version": "1.0.0",
+      "devDependencies": {
+        "@preact/signals": "^1.3.0",
+        "@shopify/ui-extensions": "^2026.1.0",
+        "preact": "^10.22.0",
+        "typescript": "^5.6.0"
+      }
+    },
     "extensions/invoice-order-action": {
       "version": "1.0.0",
       "devDependencies": {
@@ -6670,6 +6683,10 @@
       "integrity": "sha512-z1HGKcYy2xA8AGQfwrn0PAy+PB7X/GSj3UVJW9qKyn43xWa+gl5nXmU4qqLMRzWVLFC8KusUX8T/0kCiOYpAIQ==",
       "license": "MIT"
     },
+    "node_modules/customer-account-payment": {
+      "resolved": "extensions/customer-account-payment",
+      "link": true
+    },
     "node_modules/damerau-levenshtein": {
       "version": "1.0.8",
       "resolved": "https://registry.npmjs.org/damerau-levenshtein/-/damerau-levenshtein-1.0.8.tgz",
diff --git a/package.json b/package.json
index 387aaf3..edb3d05 100644
--- a/package.json
+++ b/package.json
@@ -9,7 +9,7 @@
     "deploy": "shopify app deploy",
     "config:use": "shopify app config use",
     "env": "shopify app env",
-    "start": "react-router-serve ./build/server/index.js",
+    "start": "node ./server.js",
     "docker-start": "npm run setup && npm run start",
     "setup": "prisma generate && prisma migrate deploy",
     "lint": "eslint --ignore-path .gitignore --cache --cache-location ./node_modules/.cache/eslint .",
@@ -17,7 +17,8 @@
     "prisma": "prisma",
     "graphql-codegen": "graphql-codegen",
     "vite": "vite",
-    "typecheck": "react-router typegen && tsc --noEmit"
+    "typecheck": "react-router typegen && tsc --noEmit",
+    "test": "tsx --test tests"
   },
   "type": "module",
   "engines": {
@@ -29,7 +30,11 @@
     "@react-router/dev": "^7.12.0",
     "@react-router/fs-routes": "^7.12.0",
     "@react-router/node": "^7.12.0",
+    "@react-router/express": "^7.14.2",
     "@react-router/serve": "^7.12.0",
+    "compression": "^1.8.1",
+    "express": "^4.22.1",
+    "morgan": "^1.10.1",
     "@shopify/app-bridge-react": "^4.2.4",
     "@shopify/shopify-app-react-router": "^1.1.0",
     "@shopify/shopify-app-session-storage-prisma": "^8.0.0",
diff --git a/prisma/migrations/20260515083638_add_processed_webhook/migration.sql b/prisma/migrations/20260515083638_add_processed_webhook/migration.sql
new file mode 100644
index 0000000..fa58a68
--- /dev/null
+++ b/prisma/migrations/20260515083638_add_processed_webhook/migration.sql
@@ -0,0 +1,13 @@
+-- Idempotency table for inbound Shopify webhooks. We insert a row keyed on
+-- the X-Shopify-Webhook-Id header at the start of webhook processing; a
+-- duplicate insert (P2002) means Shopify retried a delivery we've already
+-- seen, so we short-circuit and return 200 without doing the work twice.
+CREATE TABLE "ProcessedWebhook" (
+    "webhookId"  TEXT NOT NULL PRIMARY KEY,
+    "topic"      TEXT NOT NULL,
+    "shopDomain" TEXT NOT NULL,
+    "receivedAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX "ProcessedWebhook_shopDomain_topic_idx"
+    ON "ProcessedWebhook"("shopDomain", "topic");
diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index 1247b67..8ca456d 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -182,6 +182,17 @@ model EmailLog {
   @@index([shopDomain, invoiceId])
 }
 
+// Idempotency table for inbound Shopify webhooks. See
+// `app/services/webhooks/dedupe.server.ts` for details.
+model ProcessedWebhook {
+  webhookId   String   @id
+  topic       String
+  shopDomain  String
+  receivedAt  DateTime @default(now())
+
+  @@index([shopDomain, topic])
+}
+
 // Per-shop logo bytes cache. Avoids fetching the logo from Shopify Files on
 // every PDF render.
 model LogoCache {
diff --git a/server.js b/server.js
new file mode 100644
index 0000000..3ff8d74
--- /dev/null
+++ b/server.js
@@ -0,0 +1,81 @@
+// Custom production server for the React Router build.
+//
+// Replaces `react-router-serve` so we can:
+//   - prefix every console line with an ISO timestamp,
+//   - use a richer morgan format (with timestamp + content-length),
+//   - skip access logs for successful /healthz probes (they would otherwise
+//     drown out everything useful — Docker/Caddy poll them every couple of
+//     seconds).
+//
+// Behaviour is otherwise intentionally identical to `@react-router/serve`'s
+// CLI (compression, /assets immutable cache, public/ static, SIGTERM/SIGINT
+// handling).
+
+import { createRequestHandler } from "@react-router/express";
+import compression from "compression";
+import express from "express";
+import morgan from "morgan";
+
+// ---------------------------------------------------------------------------
+// Console timestamps — patch BEFORE anything else logs.
+// ---------------------------------------------------------------------------
+const ts = () => `[${new Date().toISOString()}]`;
+for (const level of ["log", "info", "warn", "error", "debug"]) {
+  const original = console[level].bind(console);
+  console[level] = (...args) => original(ts(), ...args);
+}
+
+const PORT = Number(process.env.PORT || 3000);
+const HOST = process.env.HOST;
+
+const buildModule = await import("./build/server/index.js");
+const build = buildModule.default ?? buildModule;
+
+const app = express();
+app.disable("x-powered-by");
+app.use(compression());
+
+// Static assets emitted by the React Router build.
+app.use(
+  "/assets",
+  express.static("build/client/assets", { immutable: true, maxAge: "1y" }),
+);
+app.use(express.static("build/client", { maxAge: "1h" }));
+app.use(express.static("public", { maxAge: "1h" }));
+
+// Access log: ISO timestamp + standard request info; suppress healthy
+// /healthz polls so real traffic stays visible.
+morgan.token("isotime", () => new Date().toISOString());
+app.use(
+  morgan(
+    ":isotime :method :url :status :res[content-length] - :response-time ms",
+    {
+      skip: (req, res) => req.url === "/healthz" && res.statusCode < 400,
+    },
+  ),
+);
+
+app.all(
+  "*",
+  createRequestHandler({ build, mode: process.env.NODE_ENV }),
+);
+
+const onListen = () => {
+  console.log(
+    `[server] listening on http://${HOST ?? "localhost"}:${PORT}`,
+  );
+};
+
+const server = HOST
+  ? app.listen(PORT, HOST, onListen)
+  : app.listen(PORT, onListen);
+
+for (const signal of ["SIGTERM", "SIGINT"]) {
+  process.once(signal, () => {
+    console.log(`[server] received ${signal}, shutting down`);
+    server.close((err) => {
+      if (err) console.error("[server] close error:", err);
+      process.exit(0);
+    });
+  });
+}
diff --git a/tests/dedupe-and-language.test.ts b/tests/dedupe-and-language.test.ts
new file mode 100644
index 0000000..a10409a
--- /dev/null
+++ b/tests/dedupe-and-language.test.ts
@@ -0,0 +1,79 @@
+import { strict as assert } from "node:assert";
+import { describe, it } from "node:test";
+
+import { pickLanguage } from "../app/services/invoice/i18n";
+import { isDuplicateWebhook, type DedupeDeps } from "../app/services/webhooks/dedupe.server";
+
+describe("pickLanguage", () => {
+  it("returns 'de' only for explicit German locales", () => {
+    assert.equal(pickLanguage("de"), "de");
+    assert.equal(pickLanguage("de-AT"), "de");
+    assert.equal(pickLanguage("de-DE"), "de");
+    assert.equal(pickLanguage("de_CH"), "de");
+    assert.equal(pickLanguage("DE-AT"), "de"); // case-insensitive
+  });
+
+  it("returns 'en' for non-German locales (regression: it/fr/es no longer fall back to de)", () => {
+    assert.equal(pickLanguage("en"), "en");
+    assert.equal(pickLanguage("en-US"), "en");
+    assert.equal(pickLanguage("it"), "en");
+    assert.equal(pickLanguage("it-IT"), "en");
+    assert.equal(pickLanguage("fr"), "en");
+    assert.equal(pickLanguage("fr-FR"), "en");
+    assert.equal(pickLanguage("es"), "en");
+    assert.equal(pickLanguage("hu-HU"), "en");
+  });
+
+  it("falls back to 'de' for empty/unknown input so the per-shop default chain still works", () => {
+    assert.equal(pickLanguage(undefined), "de");
+    assert.equal(pickLanguage(null), "de");
+    assert.equal(pickLanguage(""), "de");
+  });
+});
+
+function makeRequest(headers: Record<string, string> = {}): Request {
+  return new Request("https://example.com/webhooks/test", {
+    method: "POST",
+    headers,
+  });
+}
+
+function makeDeps(behaviour: "ok" | "p2002" | "boom"): DedupeDeps {
+  return {
+    db: {
+      processedWebhook: {
+        create: async () => {
+          if (behaviour === "ok") return {};
+          if (behaviour === "p2002") {
+            const err = new Error("Unique constraint failed") as Error & { code?: string };
+            err.code = "P2002";
+            throw err;
+          }
+          throw new Error("DB unavailable");
+        },
+      },
+    },
+  };
+}
+
+describe("isDuplicateWebhook", () => {
+  it("returns false on first delivery (insert succeeds)", async () => {
+    const req = makeRequest({ "x-shopify-webhook-id": "abc-123" });
+    assert.equal(await isDuplicateWebhook(req, "shop.myshopify.com", "ORDERS_CREATE", makeDeps("ok")), false);
+  });
+
+  it("returns true on a retried delivery (P2002)", async () => {
+    const req = makeRequest({ "x-shopify-webhook-id": "abc-123" });
+    assert.equal(await isDuplicateWebhook(req, "shop.myshopify.com", "ORDERS_CREATE", makeDeps("p2002")), true);
+  });
+
+  it("returns false (and proceeds) when the dedupe table itself errors \u2014 fail-open, never drop a webhook silently", async () => {
+    const req = makeRequest({ "x-shopify-webhook-id": "abc-123" });
+    assert.equal(await isDuplicateWebhook(req, "shop.myshopify.com", "ORDERS_CREATE", makeDeps("boom")), false);
+  });
+
+  it("returns false when the X-Shopify-Webhook-Id header is missing (test harness / non-Shopify caller)", async () => {
+    const req = makeRequest();
+    assert.equal(await isDuplicateWebhook(req, "shop.myshopify.com", "ORDERS_CREATE", makeDeps("ok")), false);
+  });
+});