cartToOrderSummary compatibility with the Place Order Action API
Key Takeaway
When the Commerce Webstore Checkout Place Order Action API (Streamlined Place Order) is enabled, a custom Apex call to cartToOrderSummary fails with an "Invalid checkout session status" error. Replace the call with a supported checkout API, such as the Commerce Webstore Checkout Orders API.
Sfdc_Checkout.CheckoutOrderService.cartToOrderSummary is an Apex method that custom flows can call during checkout, but it isn't supported when the Commerce Webstore Checkout Place Order Action API is enabled for a webstore. A custom flow that calls the method anyway makes checkout fail, so you need to recognize the error and know which API to use instead.
What the Place Order Action API is
The Commerce Webstore Checkout Place Order Action API is also called Place Order V2 or Streamlined Place Order. A webstore can use this API only after Streamlined Place Order is enabled for that webstore.
The Place Order Action API places an order in two steps. Both steps are sent to POST /commerce/webstores/{webstoreId}/checkouts/{activeOrCheckoutId}/orders/actions with an action of prepare or submit.
Call Prepare before payments to run validations and readiness checks. Call Submit after payments to create the order. Cart-to-order processing runs asynchronously after Submit.