Add Discount to Checkout

Adds or replaces manual discounts on an existing checkout. Each request sets the checkout's complete manual-discount state. The request is not additive: manual discounts from an earlier request that you omit from the new payload are removed. This endpoint does not remove coupons or automatic promotions. Use the request body to apply: * An order-level manual discount with `cart.discounts[].discounted_amount`. BigCommerce distributes the discount across line items based on each item's value. * An item-level manual discount with `cart.line_items[].id` and `cart.line_items[].discounted_amount`. * Order-level and item-level manual discounts together by including both arrays in the same request. Manual discounts interact with coupons and automatic promotions as follows: | Manual discount level | Coupon or automatic promotion level | Behavior | | --- | --- | --- | | Order | Order or item | The discounts stack. The result is fully additive and does not depend on which discount is applied first. | | Item | Order or item | The discounts are mutually exclusive, and the first-applied discount remains. A conflicting manual-discount request is rejected, a coupon request returns a `422` response, and an automatic promotion does not apply. | If an item-level manual discount is already applied, adding a coupon returns a `422` response with the message "Coupon `{coupon_code}` cannot be applied in combination with item level manual discount." An eligible automatic promotion does not apply while an item-level manual discount exists. The required field is `discounted_amount` at the order level, item level, or both. To prevent lost updates from concurrent requests, include the optional `version` field in the request payload. If the provided version does not match the server's cart version, the API returns a conflict error.