Leeler and Jimor bring up good points.
We use a plastic "credit card" style gift card. It has no information on it, other than its serial number. All information, including names, addresses & purchase levels are stored on the database at our offices. Were someone to counterfeit the card, they wouldn't have anything to work with, other than the current status of the account, and they wouldn't know that from looking at the card. We can block or move any "money" from one card to another, so it would be easy to deal with any lost or stolen cards.
As leeler says, the sale hasn't been made until the card is redeemed. When the gift card is purchased, the buyer is basically just moving money from his wallet to yours, without actually getting anything in return (a purchase). When he does buy something, you're moving that "money" from your wallet to the boxoffice til.
The film percentage is owing the studio for whatever performance the ticket is printed.