.

Ammm... am?

W-well... if you want to support the developer you can buy the game multiple times, hahaha.... yeah... but I don't think that any person gonna want to pay 30$ for the second time when new game update adds only 5-8MB with bug fix patch...  I mean - what kind of developers do that? (Although if think about it that way the Bethesda done crap like this when they sold the same Skyrim multiple times).

If it is already bound to itch.io. Go to "My Library" and make sure you click on the "Downloaded" button under the pitcher and not the pitcher itself. 

no you shouldn't need to buy the game again if there's an update. go to your itch.io library, and you can re-download the game with the updates there.