Steam does have rules about it, but they are not strictly enforced. If your game have it's own backend / website then you can freely bill people there after you

Steam does have rules about it, but they are not strictly enforced. If your game have it's own backend / website then you can freely bill people there after you onboarded them via Steam. IAPs are different: here Valve actually require you to process payments through steam when game is running via Steam. But even in this case a lot of games can be launched independently after installation through Steam. Also on top of this Valve already decreased it's comission from 30% for large publishers.