I would be interested in the 1600 point card ($15), but you need to step into the '90s. Concealed cash is the absolute worst way to pay for anything and I wouldn't advise it for anyone. If a postal worker gets a whiff of anything that might be cash, it will disappear off the line in a flash. Less than a flash being in Jersey. A flash mob if it's in Philly.
Have your parents open a checking account for you, then open a Paypal account with it. Done.