It's a really fun game! I especially loved the thrill of killing a huge wave of enemies right before the timer ran out and watching the remaining time shoot back up.
A few small suggestions:
I think Enter on the title screen could be replaced with Space, or Press Any Key.
It would also be nice if the shop upgrades were clickable with the mouse instead of only being selectable with the number keys.
The condition for entering the shop isn't very clear. I think it's every 30 seconds, but making that trigger more obvious would help a lot.