Really solid game, in a week jam to do this amount of systems is really cool, I liked all of them: upgrades, shop, quests, combat, etc. I have beated all the quests of the game, your game is really fun. My problems are only 3:
1 - lost items: I selled my shotgun cause I was too heavy, but I thought I could buy it again, but no, the shop now don't sell anything. When I first die, I thought my guns would be available to buy at the shop, but again... no.
2 - Flashlight: I would wish if the map was actually dark and the flashlight worked as an actual light, but actually the map is 100% visible and the zombies are invisible, the flashlight only works as a way to turn zombies visible again. That causes a fake impression that I'm in a safe place but a zombie basically spawn in my face in a sudden. If the map were actually dark, this would not be a problem.
3 - Map too big: At first I thought the map being big is amazing (actually is), but then I discovered that I had to walk for minutes to take an item and then go back all the way again, the map could be smaller, I think it would be better, but I see a potential to a big map like this, in an update you can really fill it with cool stuffs. So, keep going.
Congrats for your game, even though I said these complains, the game was really enjoyable, otherwise I wouldn't do all the quests, so congrats for doing a solid game in a short time like this.