The choice is to use the in app payment system (and pay Apple a cut) or to set up your own payment website and pay Apple nothing.

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.