Accepting Payments with Stripe

Add Stripe payments to my project using the Stripe extension:
- Create a Checkout Session for one-time payments (product: "Premium Prompt Pack", price: $9.00 USD) and redirect the customer to Stripe-hosted Checkout.
- Handle the checkout.session.completed webhook: verify the signature with the webhook secret, then grant the customer access to the purchased content.
- Show a success page after payment confirmation and a cancel page if the customer abandons checkout.
- Store payment status in canister state so access survives reloads.
- Use test mode keys during development and switch to live keys at deploy time.

import { createActor } from "@/backend";
import { useActor } from "@caffeineai/core-infrastructure";

// Frontend: request a Checkout Session URL from the canister
// (the Stripe extension signs the request server-side).
async function startCheckout(productId: string) {
  const { actor } = useActor(createActor);
  if (!actor) throw new Error("actor not ready");

  const result = await actor.createCheckoutSession({
    productId,
    successUrl: `${window.location.origin}/success`,
    cancelUrl:  `${window.location.origin}/cancel`,
    mode: "payment", // "payment" | "subscription"
  });

  if ("ok" in result) {
    // Redirect the browser to Stripe-hosted Checkout
    window.location.href = result.ok.url;
  } else {
    throw new Error(result.err);
  }
}

// Backend (Motoko): the Stripe extension exposes a typed handler.
// Verify the signature, then match on the event type.

public shared func handleStripeWebhook(
  payload : Text,
  signature : Text,
) : async Text {
  // 1. Verify signature — throws if invalid
  let event = await Stripe.verifyWebhook(payload, signature);

  // 2. Dispatch on event type
  switch (event.type) {
    case ("checkout.session.completed") {
      let session = event.data.session;
      // Fulfill the order: grant access, send email, mint NFT...
      await fulfillOrder(session);
    };
    case ("invoice.paid") {
      // Recurring subscription renewal succeeded
      await renewAccess(event.data.invoice);
    };
    case ("customer.subscription.deleted") {
      // Subscription cancelled — revoke access
      await revokeAccess(event.data.subscription);
    };
    case (_) {
      // Unhandled event — log and acknowledge
    };
  };

  return "ok";
}