The RKFL provides an API to return the cart information to display on the payment page. However, the merchant website already has the cart information to show on any page, and the RKFL API is optional.
The idea of the cart info on the payment page is similar to the below image. Please note that in the order details section, the shopper has two items including one subscription item. The RKFL supports only the exchange payment for subscription items, so the rest of the tabs are disabled.