Hey @Paolo_Carroll,
I’d suggest taking a look through this thread from last week as it contains some context on previous options and the current state: Sales Channels and multiple stores - #9 by KyleG-Shopify
TLDR; The checkout API has been deprecated and typically sales channels should use the cart API’s and cart permalinks to direct buyers to the Shopify checkout. For payment processing requests, there is the form you mentioned in the Partner Dashboard and that is the best route to discuss alternative options for your app specifically.