---

name: payment-processing description: "Use when implementing payment flows with Stripe or Polar. Covers checkout integration, webhooks, subscriptions, and error handling."

Payment Processing

Overview

Payment integration patterns for Stripe and Polar. Covers checkout flows, webhook handling, subscription management, and error handling for secure, reliable payment processing.

When to Use

• Implementing checkout flow
• Setting up Stripe/Polar integration
• Handling payment webhooks
• Managing subscriptions
• Processing refunds

Quick Reference

---

Stripe Integration

Checkout Session (Server)

Copy// app/api/checkout/route.ts
import Stripe from 'stripe';

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!, {
  apiVersion: '2023-10-16',
});

export async function POST(req: Request) {
  const { priceId, userId } = await req.json();

  try {
    const session = await stripe.checkout.sessions.create({
      mode: 'subscription', // or 'payment' for one-time
      payment_method_types: ['card'],
      line_items: [
        {
          price: priceId,
          quantity: 1,
        },
      ],
      success_url: `${process.env.NEXT_PUBLIC_URL}/success?session_id={CHECKOUT_SESSION_ID}`,
      cancel_url: `${process.env.NEXT_PUBLIC_URL}/pricing`,
      customer_email: user.email, // Pre-fill email
      metadata: {
        userId, // Track in your system
      },
    });

    return Response.json({ url: session.url });
  } catch (error) {
    console.error('Checkout error:', error);
    return Response.json(
      { error: 'Failed to create checkout session' },
      { status: 500 }
    );
  }
}

Client Redirect

Copy// components/CheckoutButton.tsx
async function handleCheckout(priceId: string) {
  setLoading(true);
  try {
    const res = await fetch('/api/checkout', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ priceId }),
    });

    const { url, error } = await res.json();
    if (error) throw new Error(error);

    window.location.href = url; // Redirect to Stripe
  } catch (error) {
    toast.error('Failed to start checkout');
  } finally {
    setLoading(false);
  }
}

---

Webhook Handling

Webhook Endpoint

Copy// app/api/webhooks/stripe/route.ts
import { headers } from 'next/headers';
import Stripe from 'stripe';

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

export async function POST(req: Request) {
  const body = await req.text();
  const signature = headers().get('stripe-signature')!;

  let event: Stripe.Event;

  // 1. Verify signature (CRITICAL)
  try {
    event = stripe.webhooks.constructEvent(
      body,
      signature,
      process.env.STRIPE_WEBHOOK_SECRET!
    );
  } catch (err) {
    console.error('Webhook signature verification failed');
    return Response.json({ error: 'Invalid signature' }, { status: 400 });
  }

  // 2. Handle event (with idempotency)
  try {
    switch (event.type) {
      case 'checkout.session.completed':
        await handleCheckoutComplete(event.data.object);
        break;

      case 'customer.subscription.created':
      case 'customer.subscription.updated':
        await handleSubscriptionChange(event.data.object);
        break;

      case 'customer.subscription.deleted':
        await handleSubscriptionCanceled(event.data.object);
        break;

      case 'invoice.payment_failed':
        await handlePaymentFailed(event.data.object);
        break;

      default:
        console.log(`Unhandled event type: ${event.type}`);
    }

    return Response.json({ received: true });
  } catch (error) {
    console.error('Webhook handler error:', error);
    return Response.json({ error: 'Handler failed' }, { status: 500 });
  }
}

Webhook Handlers

Copyasync function handleCheckoutComplete(session: Stripe.Checkout.Session) {
  const userId = session.metadata?.userId;
  const subscriptionId = session.subscription as string;

  // Idempotency check
  const existing = await db.subscription.findUnique({
    where: { stripeSubscriptionId: subscriptionId }
  });
  if (existing) return; // Already processed

  await db.subscription.create({
    data: {
      userId,
      stripeSubscriptionId: subscriptionId,
      stripeCustomerId: session.customer as string,
      status: 'active',
    },
  });
}

async function handleSubscriptionCanceled(subscription: Stripe.Subscription) {
  await db.subscription.update({
    where: { stripeSubscriptionId: subscription.id },
    data: { status: 'canceled', canceledAt: new Date() },
  });
}

---

Polar Integration

Setup

Copy// lib/polar.ts
import { Polar } from '@polar-sh/sdk';

export const polar = new Polar({
  accessToken: process.env.POLAR_ACCESS_TOKEN!,
});

Checkout with Polar

Copy// app/api/polar/checkout/route.ts
export async function POST(req: Request) {
  const { productId, userId } = await req.json();

  const checkout = await polar.checkouts.create({
    productId,
    successUrl: `${process.env.NEXT_PUBLIC_URL}/success`,
    metadata: { userId },
  });

  return Response.json({ url: checkout.url });
}

Polar Webhooks

Copy// app/api/webhooks/polar/route.ts
import { validateWebhookSignature } from '@polar-sh/sdk/webhooks';

export async function POST(req: Request) {
  const body = await req.text();
  const signature = headers().get('webhook-signature')!;

  // Verify signature
  const isValid = validateWebhookSignature(
    body,
    signature,
    process.env.POLAR_WEBHOOK_SECRET!
  );

  if (!isValid) {
    return Response.json({ error: 'Invalid signature' }, { status: 400 });
  }

  const event = JSON.parse(body);

  switch (event.type) {
    case 'subscription.created':
      await handlePolarSubscription(event.data);
      break;
    case 'subscription.canceled':
      await handlePolarCancellation(event.data);
      break;
  }

  return Response.json({ received: true });
}

---

Error Handling

Payment Errors

Copyconst STRIPE_ERROR_MESSAGES: Record<string, string> = {
  card_declined: 'Your card was declined. Please try another card.',
  insufficient_funds: 'Insufficient funds. Please try another card.',
  expired_card: 'Your card has expired. Please update your payment method.',
  incorrect_cvc: 'Incorrect CVC. Please check and try again.',
  processing_error: 'Processing error. Please try again.',
  default: 'Payment failed. Please try again or contact support.',
};

function getPaymentErrorMessage(error: Stripe.StripeError): string {
  return STRIPE_ERROR_MESSAGES[error.code ?? 'default']
    ?? STRIPE_ERROR_MESSAGES.default;
}

Graceful Degradation

Copyasync function createCheckout(priceId: string) {
  try {
    const session = await stripe.checkout.sessions.create({...});
    return { success: true, url: session.url };
  } catch (error) {
    // Log for debugging
    console.error('Stripe checkout failed:', error);

    // Return user-friendly error
    return {
      success: false,
      error: 'Unable to process payment. Please try again later.',
    };
  }
}

---

Security Checklist

• Webhook signatures verified
• Secret keys in environment variables only
• HTTPS for all payment endpoints
• Idempotency keys for critical operations
• No sensitive data in client-side code
• PCI compliance requirements met
• Rate limiting on payment endpoints

---

Common Patterns

Subscription Status Check

Copyasync function hasActiveSubscription(userId: string): Promise<boolean> {
  const sub = await db.subscription.findFirst({
    where: {
      userId,
      status: { in: ['active', 'trialing'] },
    },
  });
  return !!sub;
}

Customer Portal

Copy// Allow users to manage subscription
const portalSession = await stripe.billingPortal.sessions.create({
  customer: stripeCustomerId,
  return_url: `${process.env.NEXT_PUBLIC_URL}/account`,
});

return Response.json({ url: portalSession.url });

---

Red Flags - STOP

Never:

• Log full card numbers or CVCs
• Store payment secrets in code
• Skip webhook signature verification
• Trust client-side payment data
• Process payments without idempotency

Always:

• Verify webhook signatures
• Use server-side session creation
• Handle all webhook event types
• Log payment events for debugging
• Test with Stripe/Polar test mode

---

Integration

Related skills: api-design, nextjs-patterns Testing: Use Stripe CLI for local webhook testing