I accidentally posted this in feedback earlier instead of add-on marketplace:
^ Here has proof of concept, sample code, and even proving the ability to have a nice "add on" button in the playfab section like the others. You can develop an app from Stripe and have others connect to it to do whatever your clients want to do with them.
Works with .... everything! Excellent docs, way less sketchy than Xsolla, and everyone uses them. Even Star Citizen uses it!
See the post above for further details.