Hello! Glad you're enjoying the game.
As for the error, it definitely should not be happening. So far it has been working fine for me and lots of other people. It could either be a problem on your end or with itch.io itself, as I have no control whatsoever over the purchasing process. Try to see if it happens with others games (don't worry, you won't actually purchase them unless you go through several confirmation dialogs). If it works fine, come back to this page, hit CTRL+F5 for a full refresh, then try to buy again. If it still doesn't work, you could also try a different browser.
Please let me know how it goes!