Hello there!
It's difficult to tell what you are missing. At the moment there are about 18 paths. Not all of them have achievements or rewards attached.
I need to refine the achievement screen so that it can be used to give hints about how to unlock galleries and such. It's on my task list but not a priority.
The payment is entirely handled by Itch.io and there's nothing I can do about it. You should contact them to know more about the problem and how to solve it.