I really liked your use of the limitation! Pretty creative, and it adds a neat twist to the gameplay. The levels were fun, and I feel like the difficulty increased at a reasonable pace.
As other people have said, the shop was confusing to figure out at first (I didn't know I could cycle through by continuing to click on the options. The first time I clicked on something and it took time away, I figured I'd just permanently bought the upgrade and didn't click on any other options) but I picked up on it eventually.
Good stuff