itch.io

No, Prize Winner is MtM, Cult of the Black Feather is an F/M merge, and Deep Space Exploration has both MtM and FtF. 

The assets also have male and female alternates of the lineart in many cases

Yes, absolutely, in all cases

Ah, sorry about that. I think if it's happening all over on Itch it's probably an issue with your account, or how Itch is set up with you, and something that the Itch support team themselves would have to help you with. If it helps, I've found them pretty helpful and responsive in the past, so they should be able to get you sorted.

That's weird! I haven't heard that happening to anyone else. Is it happening if you try and purchase anything else on Itch too? 

Also, is your billing address outside the Netherlands? It looks like that's what may be causing the issue, that your IP address is saying you're in the Netherlands but the billing address you have filled in is from elsewhere.