You don't have to have any sensitive info under the scratch off under it is just a code for redeeming the coins.

http://i.imgur.com/dZQqD.jpg

As long as there is a central authority who validates the codes, you don't need the codes to be legit until time of purchase (store owner scans card to relay back that it's paid for).

Thank you for the criticism though.... I want to find any holes in this idea if possible.