1. "Create your button" seems to have a problem. When I click "Copy HTML Code" or "Copy Image Code" text changes to Copied! but actually doesn't copy anything to clipboard. Windows 10, Firefox 55, uBlock Origin. I also tried without uBlock and it's the same. I think it would be better that the code is actually shown on the form (because I can copy paste it manually then).
2. Edit page, picture choosing - I've chosen a picture which looked OK but after clicking "Save changes" the picture's width/height ratio is changed (it was streched horizontally). So I had to manually resize it in Paint.NET and then upload again. It'd be helpful if some info is added that the picture should have equal height and width.
3. During signup, after presenting the choice of Stripe and PayPal I was confused. My thought process: why I need to choose between Stripe and PayPal? Would you just generate Buy Now button? Will I have to open Stripe account? If somebody wants to donate will he have to have PayPal/Stripe also? At the end I've chosen PayPal. It seems to me this step can be skipped completely and be done after the first donation is received.
All in all, excellent job! Good luck!