Update Cart Line Item

Updates an existing, single line item in the *Cart*. **Notes** Currently, only updating `list_price` and `quantity` are supported. By default, overriding a product's list_price makes it ineligible for V3 product-level promotions. In **Store Settings** under **Promotions and Coupons** in the control panel, you can change this behavior to allow promotions on overridden prices. If the product has modifiers, omit the `variant_id` and instead use the `option_selections` array to describe both the **variant** and the **modifier** selections. If a variant needs to be changed or updated, the product will need to be removed and re-added to the cart with the correct variants using the **Add Cart Line Items** endpoint. `custom_items` cannot be updated via the API at this time. To update your cart, add a new updated custom item and delete the outdated one. If your cart contains only one line item, perform the add operation before the delete operation. Deleting all line items from the cart will invalidate the cart. To prevent lost updates due to concurrent requests overriding changes made by others, it is recommended to enable optimistic concurrency control by including the `version` field in the request payload. If the provided version does not match the version on the server, a conflict error will be returned, which the client can handle accordingly.