This is a lot of fun, although short. I think there's an error with the hitbox for the mines, though. My fire goes right through the balloon part, but the balloon part is able to damage me. Did you maybe get the hitboxes for causing/taking damage reversed?
Aside from that, I really enjoyed the progression system. It didn't feel like I was making a lot of progress until I realised that I had scored twice as much on my third run as I had on my second. consecutive runs after that, I was able to notice all the small improvements I was buying in between rounds. That's a tricky thing to balance properly, and I think you nailed it pretty well.