A very fun game, great job!
Here are a few suggestions/issues I have:
1. Upgrading an item brings me back to the first item in the list, which can get annoying
2. My gun got a little too strong and started killing monsters off-screen and I can't collect their loot. Maybe make the loot always drop inside the map even if the monsters die outside
3. Once I hit late game, it got annoying to start at level 1 again. It would be nice if there was a way/item you could buy that skips to harder levels
4. It would be cool if the dropped items had differently colored glows to represent their rarity
Overall, a very solid game. I'm actually quite interested in its development, 5/5!