Tell them you're an pro online Poker Gamer, and won them vs. Gus Hansen....
Here is what I would have done:
If your Parrents doesn't check the mail often (which mine doesn't rly) make sure you get the Mail, and only take your own letter, leave the others in there, so they dont get suspicius at all, and hide them in you room, till you feel like spend them, then make the cash and go shop xD