Payment Integration

Overview

Payment integration patterns for Stripe, Paddle, and Lemon Squeezy. Covers checkout sessions, subscription management, webhook handling, invoicing, and billing portal setup.

Capabilities

• Stripe checkout session creation
• Subscription lifecycle (create, upgrade, cancel, renew)
• Webhook signature verification and event handling
• Paddle and Lemon Squeezy integration
• Invoice generation and payment receipts
• Customer portal for self-service billing

When to Use

Trigger phrases:

• "payment integration"

• "Payment platform integration — Stripe, Paddle, Lemon Squeezy"

• Adding payments to a SaaS product

• Implementing subscription billing

• Handling payment webhooks securely

• Building a billing portal

When NOT to Use

• Task is about deployment, not development (use deploy skills)
• Task is about code review, not writing (use review skills)
• You need to understand existing code first (use research skills)
• Task is about testing only (use test skills)
• Requirements are unclear (clarify first)
• Task is trivially simple (single line fix)

Pseudo Code

The payment-integration workflow follows a standard pipeline pattern.

Core flow:

# payment-integration primary flow
input = prepare(raw_data)
result = process(input, config={billing, checkout, flows, integration, lemon})
validate(result)
deliver(result)

Error handling:

on error:
  log(error_details)
  retry_with_backoff(max=3)
  if still_failing: alert_and_escalate()

Stripe checkout session

import Stripe from 'stripe';
const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!);

// Create checkout session
const session = await stripe.checkout.sessions.create({
  mode: 'subscription',
  customer: user.stripeCustomerId,
  line_items: [{ price: 'price_pro_monthly', quantity: 1 }],
  success_url: `${BASE_URL}/billing/success?session_id={CHECKOUT_SESSION_ID}`,
  cancel_url: `${BASE_URL}/billing/cancel`,
  metadata: { userId: user.id }
});

return { url: session.url };

Webhook handler

import Stripe from 'stripe';

app.post('/webhooks/stripe', express.raw({ type: 'application/json' }), async (req, res) => {
  const sig = req.headers['stripe-signature']!;
  let event: Stripe.Event;

  try {
    event = stripe.webhooks.constructEvent(req.body, sig, process.env.STRIPE_WEBHOOK_SECRET!);
  } catch (err) {
    return res.status(400).send(`Webhook Error: ${err.message}`);
  }

  switch (event.type) {
    case 'checkout.session.completed':
      await handleCheckoutCompleted(event.data.object);
      break;
    case 'customer.subscription.updated':
      await handleSubscriptionUpdated(event.data.object);
      break;
    case 'customer.subscription.deleted':
      await handleSubscriptionCanceled(event.data.object);
      break;
    case 'invoice.payment_failed':
      await handlePaymentFailed(event.data.object);
      break;
  }

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

Paddle integration

import { Paddle } from '@paddle/paddle-node-sdk';

const paddle = new Paddle({ apiKey: process.env.PADDLE_API_KEY! });

// Create subscription
const transaction = await paddle.transactions.create({
  items: [{ priceId: 'pri_pro_monthly', quantity: 1 }],
  customerId: user.paddleCustomerId
});

// Webhook verification
app.post('/webhooks/paddle', async (req, res) => {
  const signature = req.headers['paddle-signature']!;
  const event = await paddle.webhooks.unmarshal(req.body, signature);

  switch (event.eventType) {
    case 'subscription.created': break;
    case 'subscription.updated': break;
    case 'transaction.completed': break;
  }

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

Lemon Squeezy integration

import { LemonSqueezy } from '@lemonsqueezy/lemonsqueezy.js';

const ls = new LemonSqueezy({ apiKey: process.env.LEMONSQUEEZY_API_KEY! });

// Create checkout
const checkout = await ls.createCheckout({
  storeId: process.env.LEMONSQUEEZY_STORE_ID!,
  variantId: 'variant_pro_monthly',
  customData: { userId: user.id }
});

// Webhook verification
app.post('/webhooks/lemonsqueezy', async (req, res) => {
  const secret = process.env.LEMONSQUEEZY_WEBHOOK_SECRET!;
  const signature = req.headers['x-signature']!;
  const event = ls.webhooks.verify(req.body, signature, secret);

  switch (event.meta.event_name) {
    case 'subscription_created': break;
    case 'subscription_payment_success': break;
    case 'subscription_cancelled': break;
  }

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

Common Patterns

• Customer portal: Stripe Billing Portal for self-service management
• Trial periods: subscription_data: { trial_period_days: 14 }
• Prorations: Handle mid-cycle upgrades with proration
• Idempotency: Use idempotency_key for retry-safe requests
• Webhook retries: Always return 2xx quickly, process async

How to Use

1. Understand the requirement and existing codebase patterns
2. Design the solution with error handling and testability in mind
3. Implement incrementally with tests for each change
4. Verify against expected outcomes (manual and automated)
5. Document usage, edge cases, and integration points
6. Review with team before merging to shared branches

Red Flags

• Skipping tests to ship faster: Untested code breaks in production when you least expect it
• No error handling in production code: Unhandled errors crash services and lose user data
• Hardcoded configuration values: Hardcoded values prevent environment switching and leak secrets
• Ignoring security implications: Missing input validation, auth bypasses, and injection vulnerabilities
• Over-engineering simple solutions: Premature abstraction adds complexity without proportional benefit

Verification

• Skill output matches expected behavior

Process

1. Analyze the task requirements
2. Apply domain expertise
3. Verify output quality

Anti-Rationalization Table