Royalties for online store app

Hi, if we create an App which is 3D store for goods (groceries, clothes etc) and we sell these products to users, how do we compute royalties to Epic? The app itself is free and the goods are paid for by credit card (Stripe etc). The turnover can be quite large but the profit margins very low (like 5-10% of the sale price). Any suggestions ? Thx, Sanjit

There are no royalties for revenue from physical goods bought through your app.

Thanks Newest !