Firstly you can very easily circumvent our checkout flow as we don’t lock you into using the entire system, we could therefore be supporting a large store and never receive a payment.
Secondly we see transaction fees as penalising you for selling well. It’s tough to push a product when gateways already charge for payments. A combined total of 5%+ for every order would be difficult for store owners to swallow.
We decided that offering a free tier and flat monthly fees for our paid users would be more acceptable. As a user you know exactly what you will be charged for each month.
Thanks for the feedback though, we’ll see what we can do to explain API requests and storage better.