I think the 20% is the fee for Shopify as a whole, not the payment processing fee. Since payment processing is a small portion of what they offer at Shopify its not an apples to apples comparison with Stripe.
Yeah it sucks, I just think of it as the marketing fee. I have done literally zero marketing other than put my app on their store and it has been selling for 6 years now.
As of now, the app pays for the mortgage, car payment and then some so I can't complain too much I guess.