[commoner]

As long as the cut is priced into that option (as in, the fee is passed on to the customer through a higher price for app stores) without affecting the price of buying directly from the developer, and without any anti-steering rules that prevent the developer from presenting the customer with all purchase options and prices in the app, there shouldn't be a problem.

Developers who don't want to set up payments outside of the app store can choose not to do that. But that choice should not prevent the developers who do want to offer a lower-cost payment processing option to their customers from doing so.