If I think in terms of rewarding open source contributors for the work they do, anything more than 10% (with that including credit card fees) feels extremely unfair, but if you think of it instead as providing effectively contractors for commercial projects that just happen to be open source, then higher percentages might be reasonable. I think it depends a lot on how you feel about what is happening.

Fairness is important. I've refused to use platforms because the cut seemed unfair before - it's one of the reasons I've had to stop buying kobo books through the android app- and I'm sure that's a common sentiment.