I love this!
Main thing I would've liked to see is something to make it easier to quickly compare what the customer's order to what's in my inventory. Others have brought up having the respective GUIs closer to each other, which would help. Though my first thought was some kind of indicator, like a check mark next to each individual item in a customer's order, to make it clear that the order item is currently in the inventory. So, if you have a candy corn, then there is a check mark next to the candy corn icon the customer's order. Though maybe a feature like that might eliminate too much of the challenge.