And what I would want to see is a button on the buy page that says something like add extra tipp to itch. Right now we do not know, how much the percentage is, a dev gives to itch.
I am not sure how the taxation would work, if you set the slider to 100%.
But that button above would deal with that also, since it is made clear that it is not payment for the project, but volunatry donation to the site. There are consequences for payments to a project, regardless of the percentage set. Buyers own access to that project, even if you later set a minimum price and so on.