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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.