that's an intriguing question. if there's a difference regarding taxes, shouldn't that be handled properly? maybe treat as donation if the game is free with no additional download and/or the minimum is set to $0? there are a lot of these around, so that could have a huge impact on the site (also regarding the vat handled by itchio).
(edit) digging a little, it seems donations are subject to a 60% tax on my side! the other way around then: I'd have to make it an actual sale by providing something more to people who "donate"... how does it work on itchio's side?