How to customize Stripe receipt emails (or send your own)

import express from "express";
import Stripe from "stripe";

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY);
const app = express();

// Stripe signs the raw body, so don't let a JSON parser touch this route.
app.post("/stripe/webhook", express.raw({ type: "application/json" }), async (req, res) => {
  let event;
  try {
    event = stripe.webhooks.constructEvent(
      req.body,
      req.headers["stripe-signature"],
      process.env.STRIPE_WEBHOOK_SECRET,
    );
  } catch (err) {
    return res.status(400).send(`Webhook signature check failed: ${err.message}`);
  }

  if (event.type === "invoice.paid") {
    const invoice = event.data.object;
    // Skip $0 invoices, such as the one Stripe creates when a free trial starts.
    if (invoice.amount_paid > 0 && invoice.customer_email) {
      try {
        await sendReceipt(event.id, invoice);
      } catch (err) {
        return res.status(500).send("Send failed; Stripe will retry"); // safe: the send is idempotent
      }
    }
  }
  res.sendStatus(200);
});

async function sendReceipt(eventId, invoice) {
  const amount = new Intl.NumberFormat("en-US", {
    style: "currency",
    currency: invoice.currency.toUpperCase(),
  }).format(invoice.amount_paid / 100); // divide by 1 for zero-decimal currencies like JPY
  const item = invoice.lines.data[0]?.description ?? "your subscription";

  const res = await fetch("https://api.groundmail.dev/emails", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.GROUNDMAIL_API_KEY}`,
      "Content-Type": "application/json",
      "Idempotency-Key": `stripe:${eventId}`, // a retried event can't send twice
    },
    body: JSON.stringify({
      from: "Example Billing <billing@mail.example.com>",
      to: invoice.customer_email,
      subject: `Your receipt for ${amount} (invoice ${invoice.number})`,
      text: `We received ${amount} for ${item}.\n\nInvoice ${invoice.number}: ${invoice.hosted_invoice_url}\nPDF: ${invoice.invoice_pdf}`,
      html: `<p>We received <strong>${amount}</strong> for ${escapeHtml(item)}.</p>
<p>Invoice ${invoice.number}</p>
<p><a href="${invoice.hosted_invoice_url}">View invoice</a> · <a href="${invoice.invoice_pdf}">Download PDF</a></p>`,
      tags: [{ name: "type", value: "receipt" }],
    }),
  });
  if (!res.ok) throw new Error(`Groundmail ${res.status}: ${await res.text()}`);
}