Hints in order (they get more spoilery):
Have you had Sam try taking anything out of the store? It's near the counter. See what happens when he tries to take it.
There's some cheese (I believe it's cheese) on that table.
You need to get that cheese into Whizzers container of VHS tapes.
You can't put it in there while he's looking so he needs to be distracted.
He's Whizzer after all. 
Get him to go to the bathroom and slip the cheese in while he's in there.