The supporter pack contains a few pages of concept art, character sheets, wallpapers, maps of in-game locations, and some assets from the game.
If you already donated 8$ or more through the Paper Lily itch.io page (thanks so much!) then you should be able to download the supporter pack without donating any more!
If your donation was through the Project Kat page, then it's the PK supporter pack that you already have!