Okay, the dev from that other game figured it out and I was able to buy the game via PayPal normally just now. This is what the dev discovered as the issue: "Good morning, I found the problem. My Itch.io account had the default setting to allow address sharing, which I deactivated because I'm an EU citizen. No problem though, you can now pay easily via PayPal." So, if you want, you can try that too I suppose. Up to you :)