Great little game demo, there a few things that should be improved a bit so here's a few that I don't think would be too hard to implement.

Frrst tell players they can throw items. It took me until right at the end of level 3 to figure out you can throw items. I thought a lot of dumb things because of the lack of the game telling me this. You could make a whole tutorial to teach the controls, however for now I think a simple button somewhere on the computer or menu that when clicked pops up a box telling all the control will work for a long while. Plus if you add more stuff that uses other buttons it wouldn't be hard add to it this gypothetical box.

Second the map is small. At the beginning it's perfectly sized, however after learning the game a bit and getting a few updates it become trivial to rush through it. It might be hard to add, but I think a tier system would help this problem. It would be a upgrade you can get that costs a bit, but would give you access to a new area that would be a bit bigger and have better loot. You can start this small with just adding a single tier for a update, with this eventually snowballing into a good amount of content. You can even for the first area just have it be a palate swap of the current area we go through, at least for now.

Third backpack upgrades are just better then every other upgrade. I think this would take a bit of time to fix as adding things that would cause problems that you need other upgrades, but for now I think making the backpack upgrades cost a bit and get more expensive more quickly then other upgrades. Maybe even making the other upgrades cheaper would be good too, so the player more likely to buy them over just getting more backpack upgrades.

I hope you keep developing this game as I can see it does have a great chance to be a great or maybe even amazing game later in development.