We don't offer a means to pay merchants if you're processing these payments outside of Shopify and creating orders afterward via the Orders API. The checkout API with the Sales Channel SDK's card vaulting is the way to go to have the merchant paid for their orders directly, otherwise I can't give any advice in the official capacity unfortunately.
Open to community input of course.
I know I've asked in other threds @Alex but... what about if I want to have an external marketplace that gathers different Shopify merchants (SMs) and I have a single order containing products from those different SMs? There's no way to achieve that for example... with the OrderAPI + some sort of payment flow to those SMs?
Basically I'm talking about some sort of multi SM checkout.