Build a Prepaid Email Service with Credit-Based Billing

import 'dotenv/config';
import express, { Request, Response } from 'express';
import DodoPayments from 'dodopayments';
import { Resend } from 'resend';

const app = express();

const dodo = new DodoPayments({
  bearerToken: process.env.DODO_PAYMENTS_API_KEY!,
  webhookKey: process.env.DODO_PAYMENTS_WEBHOOK_KEY!,
  environment: 'test_mode',
});

const resend = new Resend(process.env.RESEND_API_KEY!);

const CREDIT_ENTITLEMENT_ID = process.env.CREDIT_ENTITLEMENT_ID!;
const BASE_URL = process.env.BASE_URL!;

// ---------------------------------------------------------------
// Webhook endpoint MUST receive the raw body for signature
// verification. Register it BEFORE express.json().
// ---------------------------------------------------------------
app.post(
  '/webhooks/dodo',
  express.raw({ type: 'application/json' }),
  async (req: Request, res: Response) => {
    const headers = {
      'webhook-id': req.headers['webhook-id'] as string,
      'webhook-signature': req.headers['webhook-signature'] as string,
      'webhook-timestamp': req.headers['webhook-timestamp'] as string,
    };

    let event: any;
    try {
      event = await dodo.webhooks.unwrap(req.body.toString('utf8'), { headers });
    } catch (err) {
      console.error('Webhook signature verification failed:', err);
      return res.status(401).json({ error: 'invalid signature' });
    }

    switch (event.type) {
      case 'credit.balance_low': {
        const { customer_id, credit_entitlement_name, available_balance, threshold_percent } =
          event.data;
        console.log(
          `[low-balance] ${customer_id} has ${available_balance} ${credit_entitlement_name} ` +
            `left (under ${threshold_percent}%)`
        );
        await notifyCustomerLowBalance(customer_id, Number(available_balance));
        break;
      }
      case 'credit.added':
        console.log('[credit.added]', event.data);
        break;
      case 'credit.rolled_over':
        console.log('[rolled_over]', event.data);
        break;
    }

    res.json({ received: true });
  }
);

// JSON parsing for everything else.
app.use(express.json());

// ---------------------------------------------------------------
// POST /checkout/subscribe → start the MailKit subscription.
// ---------------------------------------------------------------
app.post('/checkout/subscribe', async (req, res) => {
  const { email, name } = req.body as { email: string; name: string };

  const session = await dodo.checkoutSessions.create({
    product_cart: [{ product_id: process.env.PLAN_PRODUCT_ID!, quantity: 1 }],
    customer: { email, name },
    return_url: `${BASE_URL}/?subscribed=1`,
  });

  res.json({ checkout_url: session.checkout_url });
});

// ---------------------------------------------------------------
// POST /checkout/topup → buy a 5,000-email top-up for an existing
// customer. In a real app, customer_id is resolved from the
// authenticated session, never trusted from request input.
// ---------------------------------------------------------------
app.post('/checkout/topup', async (req, res) => {
  const { customer_id } = req.body as { customer_id: string };

  const session = await dodo.checkoutSessions.create({
    product_cart: [{ product_id: process.env.TOPUP_PRODUCT_ID!, quantity: 1 }],
    customer: { customer_id },
    return_url: `${BASE_URL}/?topped_up=1`,
  });

  res.json({ checkout_url: session.checkout_url });
});

// ---------------------------------------------------------------
// GET /credits/:customerId → live balance for the dashboard widget.
// ---------------------------------------------------------------
app.get('/credits/:customerId', async (req, res) => {
  const balance = await dodo.creditEntitlements.balances.retrieve(req.params.customerId, {
    credit_entitlement_id: CREDIT_ENTITLEMENT_ID,
  });

  res.json({ balance: balance.balance });
});

// ---------------------------------------------------------------
// POST /send → send an email via Resend, then write a ledger entry
// to debit 1 credit from the customer's balance. The deduction is
// instant; the next /credits call reflects it.
// ---------------------------------------------------------------
app.post('/send', async (req, res) => {
  const { customer_id, to, subject, html } = req.body as {
    customer_id: string;
    to: string;
    subject: string;
    html: string;
  };

  // 1. Pre-flight balance check: refuse to send if the balance is at zero.
  const balance = await dodo.creditEntitlements.balances.retrieve(customer_id, {
    credit_entitlement_id: CREDIT_ENTITLEMENT_ID,
  });

  if (Number(balance.balance) <= 0) {
    return res.status(402).json({
      error: 'No email credits remaining. Buy a top-up pack or upgrade your plan.',
    });
  }

  // 2. Send via Resend.
  const { data, error } = await resend.emails.send({
    from: 'MailKit <onboarding@resend.dev>', // swap for your verified domain
    to: [to],
    subject,
    html,
  });

  if (error) {
    return res.status(500).json({ error: error.message });
  }

  // 3. Debit 1 credit. Resend's message ID is the idempotency key: if this
  //    ledger call is repeated for the same send, Dodo Payments rejects the
  //    duplicate with 409 Conflict instead of debiting the customer twice.
  await dodo.creditEntitlements.balances.createLedgerEntry(customer_id, {
    credit_entitlement_id: CREDIT_ENTITLEMENT_ID,
    amount: '1',
    entry_type: 'debit',
    reason: `email send ${data!.id}`,
    idempotency_key: data!.id,
  });

  res.json({ id: data!.id });
});

async function notifyCustomerLowBalance(customerId: string, available: number) {
  // In production: send an email to the account owner, push a banner,
  // open an in-app modal, etc. For the demo we just log.
  console.log(`[NOTIFY] ${customerId}: ${available} emails left. Consider topping up.`);
}

app.use(express.static('public'));

const port = Number(process.env.PORT) || 3000;
app.listen(port, () => {
  console.log(`MailKit running on http://localhost:${port}`);
});

<!doctype html>
<html>
  <head>
    <title>MailKit Demo</title>
    <style>
      body {
        font-family: system-ui, -apple-system, sans-serif;
        max-width: 720px;
        margin: 40px auto;
        padding: 0 20px;
        color: #1a1a2e;
      }
      h1 { font-size: 28px; margin-bottom: 4px; }
      h2 { font-size: 16px; margin-top: 32px; padding-bottom: 6px; border-bottom: 1px solid #eee; }
      label { display: block; font-size: 13px; font-weight: 600; margin: 12px 0 4px; }
      input, select, textarea {
        width: 100%;
        padding: 10px;
        border: 1px solid #ddd;
        border-radius: 6px;
        font-family: inherit;
        font-size: 14px;
        box-sizing: border-box;
      }
      button {
        background: #1a1a2e;
        color: white;
        padding: 10px 18px;
        border: none;
        border-radius: 6px;
        cursor: pointer;
        font-size: 14px;
        margin-top: 12px;
      }
      button:hover { background: #2d2d4a; }
      .out {
        background: #f6f6fa;
        padding: 12px;
        border-radius: 6px;
        margin-top: 12px;
        font-size: 13px;
        font-family: ui-monospace, monospace;
        white-space: pre-wrap;
        word-break: break-all;
      }
      .balance { font-size: 36px; font-weight: 700; color: #4f46e5; }
      .balance-sub { color: #888; font-size: 13px; margin-top: 4px; }
    </style>
  </head>
  <body>
    <h1>MailKit</h1>
    <p>Prepaid transactional email, billed per send.</p>

    <h2>1. Subscribe to MailKit ($19/mo, 5,000 emails)</h2>
    <label>Email</label>
    <input id="subEmail" type="email" placeholder="you@example.com" />
    <label>Name</label>
    <input id="subName" type="text" placeholder="Your name" />
    <button onclick="subscribe()">Get checkout link</button>
    <div id="subOut" class="out" hidden></div>

    <h2>2. Check your balance</h2>
    <label>Customer ID</label>
    <input id="balCust" type="text" placeholder="cus_xxxxxxxxxxxx" />
    <button onclick="checkBalance()">Refresh</button>
    <div id="balOut" class="out" hidden></div>

    <h2>3. Send a transactional email</h2>
    <label>Customer ID</label>
    <input id="sendCust" type="text" placeholder="cus_xxxxxxxxxxxx" />
    <label>To (Resend's sandbox accepts delivered@resend.dev)</label>
    <input id="sendTo" type="email" value="delivered@resend.dev" />
    <label>Subject</label>
    <input id="sendSubj" type="text" value="Hello from MailKit" />
    <label>HTML body</label>
    <textarea id="sendBody" rows="3">&lt;strong&gt;It works!&lt;/strong&gt;</textarea>
    <button onclick="sendEmail()">Send</button>
    <div id="sendOut" class="out" hidden></div>

    <h2>4. Run low? Buy a top-up pack</h2>
    <label>Customer ID</label>
    <input id="topCust" type="text" placeholder="cus_xxxxxxxxxxxx" />
    <button onclick="topup()">Buy 5,000 emails ($9)</button>
    <div id="topOut" class="out" hidden></div>

    <script>
      const show = (id, content) => {
        const el = document.getElementById(id);
        el.hidden = false;
        el.innerHTML = content;
      };

      async function subscribe() {
        const r = await fetch('/checkout/subscribe', {
          method: 'POST',
          headers: { 'Content-Type': 'application/json' },
          body: JSON.stringify({
            email: document.getElementById('subEmail').value,
            name: document.getElementById('subName').value,
          }),
        });
        const data = await r.json();
        show('subOut', r.ok
          ? `<a href="${data.checkout_url}" target="_blank">Open checkout →</a>`
          : `Error: ${data.error}`);
      }

      async function checkBalance() {
        const id = document.getElementById('balCust').value;
        const r = await fetch(`/credits/${id}`);
        const data = await r.json();
        show('balOut', r.ok
          ? `<div class="balance">${Number(data.balance).toLocaleString()}</div>
             <div class="balance-sub">emails available</div>`
          : `Error: ${data.error}`);
      }

      async function sendEmail() {
        const r = await fetch('/send', {
          method: 'POST',
          headers: { 'Content-Type': 'application/json' },
          body: JSON.stringify({
            customer_id: document.getElementById('sendCust').value,
            to: document.getElementById('sendTo').value,
            subject: document.getElementById('sendSubj').value,
            html: document.getElementById('sendBody').value,
          }),
        });
        const data = await r.json();
        show('sendOut', r.ok ? `Sent. Message id: ${data.id}` : `Error: ${data.error}`);
      }

      async function topup() {
        const r = await fetch('/checkout/topup', {
          method: 'POST',
          headers: { 'Content-Type': 'application/json' },
          body: JSON.stringify({ customer_id: document.getElementById('topCust').value }),
        });
        const data = await r.json();
        show('topOut', r.ok
          ? `<a href="${data.checkout_url}" target="_blank">Open top-up checkout →</a>`
          : `Error: ${data.error}`);
      }
    </script>
  </body>
</html>