Uhm. You even promised so on your itch page.
If I purchase the game here on itch.io, will get a Steam key so I don't have to do it again? Yes, everyone who purchases the game on itch.io will be given access to a Steam key and will have full access to a paid, updated version of Milo forever.
If you really made a sale with 100% discount, what did you think would happen?
You might want to read this too https://itch.io/docs/creators/download-keys
Well, it might give your game a little kickstart, once it releases.