Hey there!
I'm a Premium donator, and currently have 59 items in my store. I have just tried to access my store from the My Shop menu, only to be greeted with a 'Your shop is full!' error message. I thought to myself, "Hang on, this can't be right", and so I looked down to my inventory, and lo and behold, I have used 59/50 slots, the same limit for Advanced donators.
Is this just a bug, or have the item limits changed?
