The paid version will unlock if you pay $15 or more or buy the game on Steam.
I'm very glad to hear that! As for a preference in buying, Steam takes a larger cut of the gross sales compared to itch.io, so buying it on itch.io is a little more profitable for me. The Steam version has achievements though, which the itch.io version lacks. If achievements are important to you, the Steam version is the way to go.