It's an interesting concept, and I like the "meta" aspect of upgrading the game that the character is working on, which is the game that the player is playing, and which thus upgrades the latter, too. ^_^
I do think that the "stock room" area could perhaps better convey that an item has been selected: there isn't much feedback there, and in particular no visible change of a given item's "state". It's only on going back to the till that one discovers that an item has been successfully selected. This can make for a minor bump in the experience at times, I think.
(I'm also honestly not clear on what the toy located second from the right on the second shelf might be. Looking at it again now, I'm guessing that it's a toy car? ^^; )
Conversely, however, figuring out which toy a customer wants is a nice little challenge! ^_^