Yes, it's a limitation of this platform. We don't understand why in a place where you "buy" the games, accumulative payments is not the normal thing.
But don't worry. We can check the payments of most of the users. And we'll give access keys to the game to those who just pay the difference between their previous payments and the full price of the game. I just checked you already paid 7$.
Reach us to taifunriders@gmail.com or on Discord.
https://discord.gg/VfykcYhxuw
There, we'll give you the access key (if you are interested in this option) after checking the remaining amount was paid. It's the most we can do in this situation and giving how Itch.io works