I found the page that describes how! https://itch.io/docs/buying/already-bought
Check the section called "How to download the game, or get your activation key". You just need to find your way back to the downloads page. You should be eligible for a key even by getting it through the bundle.
I also recommend getting the Itch desktop app. It makes managing your game library and installations waaay easier, for games that you only have on itch. https://itch.io/app
Let me know if you have any other complications!