Nice game! I have some ideas for you.
1. There should be some custom game options like number of customers, rudeness of customers, boxes per aisle, how big the store is, more employees in the store, time to complete, etc. Or to make it easier, there should be different difficulty settings. Easy, Normal (what this is already), hard, extreme, impossible.
2. The aisles should be more customized to that particular aisle, like the clothing aisle is.
3. If you stand still or move very little for 5 seconds when your not stocking, you get 1 strike. When you get 3 strikes, your fired.
4. There should be a story mode to the game (the mode that the game currently is could be a practice mode, with the settings I mentioned earlier) and the difficulty of the game increases. Lets say for the sake of discussion, the game takes place in August (retail stores are very early to the holidays). Later levels could have more customers, ruder customers, etc.
Hope these ideas make it into the game!