If Graham were 5, I could even appreciate his logic.
At 10, he surely knows this is cheating, and has demonstrated he knows by stuffing the receipts that aren't his behind a display rather than leaving them where anyone could see them.
What Graham hasn't won is the bear. He has disqualified himself from the drawing. Assuming that the other receipts are accessible and not destroyed, they can be put back into the box for a drawing, less my/Graham's receipts. If the shop owner is amenable, Graham can participate by choosing the winner--who is guaranteed NOT to be him. I know that's "mean"--but it's also instructive.
If the (removed) receipts have been destroyed, I'm going to suggest they start the contest over, this time with a locked box.
Graham and I are going to talk about this issue. If he can convince me he really didn't know what he was doing was wrong (which I doubt), we will leave it at talk. Otherwise, it's time for the hairbrush to do the talking.