This is not true; at least, you cannot redirect within the app. Spotify had its updates rejected when it had a flow that ended all IAPs, and redirected people to a web browser.
The kindle app offers no way to purchase for this reason.
The kindle app offers no way to purchase for this reason.
Neither Google or Apple do that.
Of course, even if you set up your own payment infrastructure you are going to be paying somebody a cut.
Visa, MasterCard, PayPal, and Stripe et al. are going to take a cut of any transactions they process.
You have to decide if the fee they charge is worth the additional sales accepting that method of payment might bring in.