Afraid there is no Steam key included here, if you want to play the game on Steam then purchase it on Steam. Sorry about double paying, but it’s like any other game over multiple platforms.
Viewing post in Shattered Pixel Dungeon comments
I double checked to remind myself and this is due to Shattered being a ‘pay what you want’ game on itch.io. Due to the way itch’s system works, ANY payment, even $0.01, would entitle someone to a Steam key. I can’t set a minimum payment threshold like I can for the separate ‘paid’ files you can download directly from itch. Because of this the only way to offer Steam keys here would be to also remove the free version and only charge $10 up-front.