@kaliju I appreciate all about that. "Create invoice" API needs to be authorized through authentication for rate contol and the security reason on the backend. The previous SteemPay could be easily exploited aa well. You can make the similar pay without API call and the database. But then it will bring in another security issue which is there is no way to verify the QR code is compromised or not.
Once this service lose any credit about security the entire project will never survive because this is a "payment" app. I like your enthusiasm on SteemPay and the vision you see through. So how about finding out a better way to appeal the sellers to sign up?