Very happy you liked our little game. This was a jam project we are still working on (albeit slower than anticipated). The shop is one of the weakest points and we will eventually move it out of the battle and into the world map. Also, everything is only one coin which we also need to balance and price properly. And we have plans for many more upgrades and unique cards in the shop, we will make sure to post a devlog when we do that.
I need to take a look at the bug your mentioned. Is it possible you had the cards drawn already before you opened the shop and bought the upgrade? Drawn cards may not update properly (another reason to move the shop out of battle, but I can patch it for now).
Thanks for playing.
