I'm not a dev, but i'm just curious on how you spent money on this and then didn't get the game? it's 5USD to purchase, so you spent less than that? the price is usually automatic with things like this too so you edited it to be lower than the price for the full version then was disappointed when you didn't get it?
the reason it allows you to pay less than the price it costs for the full version is to give donations, which is what you unintentionally did. it's kind of on you and you can't really expect the devs to do something about that, unless i've missed something and you did pay the proper price, in that case it's an issue with itch.io not the game itself as it should automatically give you the game upon purchasing of it.