Earlier quoted context omitted.
~3% or less.
~3% plus some fixed transaction fee. So for low dollar amount charges, like a $1.00 app purchase, the transaction fee can be > 30% (e.g. 2.9% + $0.30 on Stripe).
And for that stripe fee you still have to have your own store, download infrastructure, and website to sell it on. A website that you also have to build, maintain, and market. And you have to deal with chargebacks and customer support during the buying process.