Thanks so much for replying! I did redeem the key on Steam so I'll just grab the DLC on there if there's not a solution... but I do vastly prefer using Itch :). Maybe I'll just wait to see if you can't find a way? I don't want to just NOT purchase the DLC, but at the same time I would rather support Itch, hope that makes sense!
There's an Itch page for the DLC now :) - Also includes a Steam key! https://fallahn.itch.io/super-video-golf-adventurer-dlc