Payments created but not reflected on WebCart
Key Takeaway
When a payment succeeds but isn't saved to the WebCart, look for concurrent cart updates and make sure the payment waits until GET /checkouts returns 200 OK.
In B2B Commerce checkout, a payment can process successfully but never get saved to the WebCart. Without the saved payment, the order can't be created. The payment itself succeeds, so the payment step can look healthy, but the real problem is the save back to the WebCart.
Concurrent modification
The most frequent cause is concurrent modification: custom code or other endpoints update the cart state at the same time the Checkout APIs are working on it. The conflict works like two people saving edits to the same file from different copies.
A storefront has custom Apex that updates a custom field on WebCart, Delivery_Notes__c, when the buyer clicks "Place Order". The Payment API loads the cart at version 12. The custom Apex then saves version 13. When the Payment API tries to save using the old version 12, the system detects the mismatch. The system rejects the save with "Looks like your cart is currently in use. Please try again later."
The "cart is currently in use" error means multiple concurrent calls are trying to update cart state.