Create a BigCommerce Storefront cart via GraphQL

Steps

1. Obtain a Storefront API JWT token scoped to the appropriate channel (see the token generation route above) and include it as Authorization: Bearer {token} on GraphQL requests.
2. Send a GraphQL mutation to https://store-{hash}.mybigcommerce.com/graphql using the createCart mutation, providing an input object with lineItems — each item specifying productEntityId, variantEntityId (if applicable), and quantity.
3. The mutation returns the cart object including entityId (the cart UUID), lineItems, amount, and currencyCode — store the entityId for subsequent cart operations.
4. To add items to an existing cart, use the addCartLineItems mutation with the cartEntityId and additional lineItems input; to update quantity use updateCartLineItem.
5. Redirect the customer to the checkout by constructing the URL: https://{storefront-domain}/cart.php?action=loadInCheckout&id={cart-entityId} or use the redirect URLs returned by the checkout API.