Create a BigCommerce headless checkout flow using the REST Management API from cart creation through order placement

Steps

1. Create a cart with POST /v3/carts using the Store Management API (X-Auth-Token header); include line_items array with product_id, variant_id, and quantity
2. Create a checkout from the cart by calling GET /v3/checkouts/CART_ID — BigCommerce creates a checkout object automatically when a cart is fetched via this endpoint
3. Add a billing address with POST /v3/checkouts/CHECKOUT_ID/billing-address supplying address object fields including first_name, last_name, address1, city, state_or_province, postal_code, country_code, and email
4. Add a consignment (shipping address + line item mapping) with POST /v3/checkouts/CHECKOUT_ID/consignments; the response includes available shipping_options for the address
5. Select a shipping option with PUT /v3/checkouts/CHECKOUT_ID/consignments/CONSIGNMENT_ID with shipping_option_id from the previous response
6. Create an order from the finalized checkout with POST /v3/checkouts/CHECKOUT_ID/orders; process payment separately using the Payments API with POST /v2/payments