For a smaller, not as competitive but still well-knownish school here's the trick:
Don't return the money or sign anything. They WILL follow up. It's easier to keep the body you have rather than go out and find a new body to fill a spot. Also it looks better for retention rates so they'll want to keep you.
When they follow up wait up to 24 hours and then send an email saying "yes, we just forgot" or "omg hubby took the checkbook and I couldn't leave a deposit" and then tell them you will send it in a backpack.
Then don't send it in for another day or two. That should buy you enough time.