Resend Invoice Demo

A Node.js invoice email service that generates PDF invoices and sends them via Resend. Built as an interview demo project to showcase Resend's transactional email, scheduling, and webhook capabilities.

Features

• PDF invoice generation with PDFKit
• Transactional email via Resend with PDF attachment
• Optional scheduled receipt email (using Resend's scheduled_at)
• Webhook handling for delivery events (delivered, bounced, complained)
• Svix signature verification for webhooks

Prerequisites

1. A Resend Account. Sign up here.
2. A Resend API Key.
3. A real email to send from.

Emails to use

• onboarding@resend.dev can be used for the sender email. The resend.dev domain is available for testing, but with a restriction: it can only send to your own email address.
• Resend provides test addresses like delivered@resend.dev, bounced@resend.dev, complained@resend.dev, and suppressed@resend.dev to simulate different delivery scenarios.

Setup

Package manager: pnpm is recommended; npm can also be used.

git clone <repo>
cd resend-demo
pnpm install   # or: npm install
cp .env.example .env
# Edit .env with your values
pnpm start     # or: npm start

Health Check

Confirm the server is working by running:

curl http://localhost:{port}

A successful response will look like:

{"status":"ok"}

API Usage

Send an Invoice

Set the clientEmail to a real email address.

curl -X POST http://localhost:{port}/invoice \
  -H "Content-Type: application/json" \
  -d '{
    "lineItems": [
      { "description": "Consulting", "quantity": 5, "rate": 150 },
      { "description": "Design review", "quantity": 2, "rate": 200 }
    ],
    "clientName": "Acme Corp",
    "clientEmail": "you@youremail.com",
    "schedule_receipt": true,
    "delay_minutes": 1
  }'

Use delay_minutes to control when the payment receipt is sent. Defaults to 1 if omitted.

Field	Type	Default	Description
delay_minutes	number	1	Minutes to wait before sending the payment receipt email

Response:

{
  "success": true,
  "invoiceId": "INV-20260217-4823",
  "invoice_total": 1150,
  "from": "Resend <onboarding@resend.dev>",
  "to": "you@youremail.com"
}

---

Endpoints

Method	Path	Description
GET	/	Health check
POST	/invoice	Generate and send invoice email
POST	/webhooks/resend	Receive Resend delivery events

---

Webhooks

Resend uses Svix to sign webhook payloads. Every request to POST /webhooks/resend includes svix-id, svix-timestamp, and svix-signature headers. The server verifies these headers against WEBHOOK_SIGNING_SECRET before processing any event.

Events

Subscribe to these in the Resend dashboard when adding your webhook endpoint:

Event	Description
email.bounced	Permanently rejected by recipient server
email.clicked	Recipient clicked a link in the email
email.complained	Recipient marked as spam
email.delivered	Successfully delivered to recipient's mail server
email.delivery_delayed	Temporary delivery issue (inbox full, server transient)
email.failed	Send failed (invalid recipient, API issues, etc.)
email.opened	Recipient opened the email
email.received	Resend successfully received the email
email.scheduled	Email scheduled to be sent
email.sent	API accepted the request
email.suppressed	Email suppressed by Resend

Getting the Signing Secret

1. Open the Resend Dashboard
2. Go to Webhooks → Add Endpoint
3. Enter your endpoint URL (e.g. https://your-domain.com/webhooks/resend)
4. Copy the Signing Secret shown after creation
5. Add it to your .env:

   WEBHOOK_SECRET=whsec_...
   

401 Invalid signature? The signing secret is unique per webhook endpoint. If you changed your ngrok URL, you created a new endpoint—use the secret from that endpoint. No spaces/newlines when copying.

Use the port you've set in .env

ngrok http {port}

Use the port your app runs on (see PORT in .env). Add the HTTPS URL + /webhooks/resend as your webhook endpoint in the Resend dashboard.

Project Structure

src/
  index.js      # Express app and routes
  invoice.js    # PDF generation (generateInvoicePDF, generateInvoiceId)
  email.js      # Email sending (sendInvoiceEmail, scheduleReceiptEmail)
.env.example    # Environment variable template