Payments Expert - Stripe, PayPal, Connect

Comprehensive payment processing expertise for Stripe, PayPal, and marketplace payments.

Stripe Checkout (Quick Start)

import stripe

session = stripe.checkout.Session.create(
    payment_method_types=['card'],
    line_items=[{
        'price_data': {
            'currency': 'usd',
            'product_data': {'name': 'Premium'},
            'unit_amount': 2000,  # $20.00
        },
        'quantity': 1,
    }],
    mode='payment',  # or 'subscription'
    success_url='https://example.com/success?session_id={CHECKOUT_SESSION_ID}',
    cancel_url='https://example.com/cancel',
)
# Redirect to session.url

Webhook Handling

@app.route('/webhook', methods=['POST'])
def webhook():
    event = stripe.Webhook.construct_event(
        request.data,
        request.headers.get('Stripe-Signature'),
        webhook_secret
    )

    if event['type'] == 'checkout.session.completed':
        session = event['data']['object']
        await confirm_payment(session.id)

    return 'Success', 200

Critical Events

Stripe Connect (Marketplaces)

Charge Types

Direct Charge Webhook Setup

# /webhooks/stripe/connect - CRITICAL for Direct Charge!
@app.route('/webhooks/stripe/connect', methods=['POST'])
def connect_webhook():
    event = stripe.webhooks.constructEvent(
        request.data, sig, connect_webhook_secret
    )

    if event['type'] == 'checkout.session.completed':
        # Direct Charge sessions complete here!
        await handle_connect_checkout(event.data.object)

Critical Patterns

Dual Confirmation (Webhook + Frontend)

// Both webhook and frontend call this - idempotent!
async function confirmPayment(sessionId: string) {
  const result = await db.update(orders)
    .set({ status: 'paid' })
    .where(and(
      eq(orders.stripeSessionId, sessionId),
      eq(orders.status, 'pending')  // Only if pending!
    ));

  if (result.changes === 0) return false; // Already processed
  await decrementInventory(sessionId);
  return true;
}

100% Promo Code Detection

# ✅ CORRECT - amount_total=0 + payment_status='paid'
def is_100_percent_promo(session):
    return (
        session.payment_status == 'paid' and
        session.amount_total == 0 and
        session.payment_intent is None
    )

# ❌ WRONG - no_payment_required is different!

Web vs Native Browser Handling

// WebBrowser result types differ by platform!
switch (result.type) {
  case 'dismiss':  // Native: browser closed - NOW verify
    await verifyPayment(sessionId);
    break;
  case 'opened':   // Web: user still on Stripe page - DON'T verify yet!
    Alert.alert('Complete payment in browser');
    break;
}

Subscriptions

subscription = stripe.Subscription.create(
    customer=customer_id,
    items=[{'price': price_id}],
    payment_behavior='default_incomplete',
    expand=['latest_invoice.payment_intent'],
)

PayPal Integration

// PayPal SDK
paypal.Buttons({
  createOrder: (data, actions) => {
    return actions.order.create({
      purchase_units: [{ amount: { value: '20.00' } }]
    });
  },
  onApprove: async (data, actions) => {
    const order = await actions.order.capture();
    await verifyPayment(order.id);
  }
}).render('#paypal-button');

Testing

TEST_CARDS = {
    'success': '4242424242424242',
    'declined': '4000000000000002',
    '3d_secure': '4000002500003155',
}

Best Practices

1. Always use webhooks - Don't rely on frontend only
2. Idempotency - Handle duplicate events
3. Verify signatures - Always validate webhooks
4. Never store raw card data - Use Stripe Elements
5. Implement SCA - 3D Secure for Europe

Related Skills

• billing - Subscription management
• compliance - PCI compliance