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

Event	Use Case
checkout.session.completed	Payment successful
payment_intent.payment_failed	Payment failed
customer.subscription.updated	Subscription changed
account.updated	Connect account status

Stripe Connect (Marketplaces)

Charge Types

Type	Who Creates	Webhook Location
Direct	Connected Account	Connect endpoint
Destination	Platform	Platform endpoint

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