I have a big free interactive fiction text adventure that has been available without cost for a few months.

It is similar in size to the game Hadean Lands. On Steam, Hadean Lands has a DLC that's just a certificate you can sign to brag that you beat the game without using hints. It's mostly a fun way to show support to the author.

If I hosted my game on itch, would I be allowed to follow this model and have paid pdfs for download?  The description would make clear what they are, and I wouldn't want anyone to buy them by accident thinking it's a game.

Similarly, could I put a goal on the page for things like, 'if this certificate makes $700, custom portraits of each character will be added to the game' or 'everyone who buys this certificate will have their name added to this page?'

I think this kind of thing could be fun, but I could also see it being against itch rules, so I wanted to check first!