Alternative payment options for IAPs

I am developing a VR title that is free to play with in-app purchases. I was wondering if anyone had experience using a third party IAP provider other than Steam/Oculus, while distributing the app through those platforms?

The main motivation is to have a single payment processor instead of two ( or more ).

First, is it permitted by Oculus/Steam?

Second, if you have experience doing this, perhaps you could comment on a few of the pros-cons?

Thx

Ping... anyone have info on this topic?