Implement BigCommerce Embedded Checkout in a headless storefront using the Checkout SDK embedCheckout method

Steps

1. Create a cart via the BigCommerce REST Storefront API POST /api/storefront/carts or the REST Management API POST /v2/carts, returning a cartId
2. Retrieve the checkout URL from GET /api/storefront/checkouts/CART_ID or /v3/checkouts/CART_ID; the checkoutUrl points to the BigCommerce-hosted checkout page
3. On your headless frontend, load the Checkout SDK and call embedCheckout({ containerId: 'checkout-container', url: checkoutUrl }) to render the checkout in an iFrame inside your page
4. Provide a styles object to embedCheckout() to pass custom CSS variables that BigCommerce injects into the hosted checkout iframe for basic brand alignment
5. Listen for checkout events by passing an onComplete callback in the embedCheckout config options to detect when the customer completes the order
6. Ensure your domain is added to the BigCommerce store's allowed CORS origins in Settings > Advanced > Allow List to prevent iframe cross-origin errors