My main issue was understanding the shop menu. I didnt get how to buy upgrades at all. I tried clicking on the individual upgrades trying to unlock them, since I it seemed like my coin count would be sufficient. But nothing happened when I click on them. At some time I figured out I can buy ugprades with pressing Enter, but that was confusing since I dont even know what upgrades have been bought.
The hard to read font text also made it more difficult than it had to be. I tried to open the shop with F all the time since it looked like an F key, but it was E instead :D