I found yesterday that the latest Api is now supporting custom POS for applying AppFee. This is really great update. I am a custom POS developer, also manages a fine dine-in restaurant.
Square is good.
Custom POS can only use Square Terminal but using terminal is very enough for custom POS. Custom POS developer may have an issue on dupe-check for transaction.
As a developer, I will share my coding on this issue if you ask me.