OK, a couple of comments. While the game is enjoyable, it really could use some quality of life changes.
First, an option to hold to fire instead of needing to constantly click to shoot would be good.
Second, there should be a delay between when the level up options appear and when you can click on them. Something like a 1/3 second if the cursor was over the button when the level up triggered. That way you aren't trying to shoot something and accidentally picked a car you don't want.
Third is more opinion based, I personally think pausing and letting the player add / combine cars during the level up would be better than the current inventory system. This one is mostly just my opinion, but having the level up and then needing to separately open the inventory to install the upgrade isn't particularly good IMO.