No, I mean, how I know that the person purchased the game on Steam, so I can distribute the keys automatically for them. Something like with that .CSV file system of Kickstarter.
Err... you have to figure that out on the Steam side?
Ouch! They don't have any system to let us know who purchased the game, as far I know… -_-'
I don't know anything about selling on steam but I bet you could put a menu option in your game that ties in with the Steam API and requests an itch key from your own server.
That's a good idea, but I removed my games from Steam, I don't know if their API still works for them.
Also, I have a RPG Maker 2003 game that doesn't have support to the Steam API anyway. =/