Checkout API allows for a checkout to be created from an existing cart using BigCommerce checkout logic. The existing BigCommerce front-end cart/checkout can be bypassed.
The Checkout API makes it possible to purchase products from a BigCommerce store programmatically. You also have the option to use the GraphQL Storefront API to build an external checkout in your CMS or app.
You can retrieve a checkout by sending a GET request to the /checkouts endpoint and including the checkout_id.
Cart_id and checkout_id are the same Id.
You can add a billing address to an existing checkout by sending a POST request to /checkouts/{checkout_id}/billing-address
Required Fields:
emailcountry_codeBody Example

Update an existing billing address with a PUT request to /checkouts/{checkout_id}/billing-address/{address_id}
Body Example

A consignment is a list of physical products that will travel together to the purchaser, and it specifies how those items can and will ship. It is an object that includes at least one product line item and one shipping address.
Add a new Consignment to Checkout by sending a POST request to /checkouts/{checkout_id}/consignments
Required Query:
consignments.available_shipping_optionsRequired Fields:
shipping_addressline_itemsUpdates an existing consignment. Shipping address, line item IDs or the shipping option ID can be updated using this endpoint.
Required Fields:
shipping_option_idThere are two steps to add a new shipping address and shipping options with line items.
include=consignments.available_shipping_options to return the available shipping options based on line items and shipping locations. This will return available_shipping_options in the response.shipping_option_id (shipping address and line items) with the available_shipping_option id from Step One.
Removes an existing consignment from checkout.
Add a Coupon Code to a checkout with a POST request to /checkouts/{checkout_id}/coupons
Required Fields
coupon_codeDeletes a Coupon Code from Checkout.
To add a discount to an existing checkout, send a POST request to /checkouts/{checkout_id}/coupons
In the request body, you need to include the following information:
couponCode: The code of the discount that you want to add.couponType: The type of discount that you want to add. Valid values are percentage_discount and amount_discount.discountedAmount: The amount of the discount. For a percentage discount, this is the percentage of the checkout total that will be discounted. For an amount discount, this is the amount of money that will be discounted.For example, the following request would add a 10% discount to a checkout with the ID 12345:
Here are some additional things to keep in mind about adding discounts to checkouts:

Create an order by sending a POST request to /checkouts/{checkout_id}/orders