Hi hello!
As of writing this, I'm riight at the end of the game.
So, I've read that other person's message saying that upgrades makes no sense, along with everything else in the game, so I was a bit apprehensive.
Thankfully my fears were unanswered, because it seems like you only needed to read the tooltips to understand the game (and also look what upgrades does what, pretty simple really).
Anyways, I'm exited to see how "cursed" this ending will be! (according to some other comment, and the Poem in the description)
Cheers!
(PS : There's also a small problem you might want to fix: some of the last 10-15 Zap upgrades have the wrong colour/price shown compared to the true price (some of the other upgrades might also have this problem))