Mostly giving loot chests and such, Amazon Prime customers with linked Twitch accounts (that is, Twitch Prime customers) sometime get full game offers.
Layers of Fear: Masterpiece Edition is currently $22.99 on Steam.
How to get:
1) Link Amazon Prime and Twitch accounts
2) Click crown symbol at top of most twitch.tv pages (like here)
3) Click "Claim Offer" button under Layers of Fear game (can collect other loot same way if you wish)
4) Follow steps they give you to download, install, and run Twitch desktop app