itch.io support page says that for Among Us, you should contact the devs at us@innersloth.com but recent messages from Innersloth have said to use support@innersloth.com - I contacted the first email address 5 days ago and haven't seen any reply beyond an automated "message received" notification. I sent an email to the address Innersloth have provided yesterday.
Steam support won't do anything for you regarding a purchase outside of their platform and would, like itch does, direct you to contact the devs first.
I'm going to hope I hear from the devs soon, but if not, I'll be contacting itch.io support directly and getting a refund (since I don't have access to Steam for medical reasons).