Well, in case of retail cards the keys are registered before shipping. You give the manufacture a set of registered keys, which are then printed on the cards. I

Well, in case of retail cards the keys are registered before shipping. You give the manufacture a set of registered keys, which are then printed on the cards. If done right you won't have any issues - that's exactly what people are doing with CD-Keys or Prepaid-Cards. However, if you mess up (can't always avoid mistakes) and have shipped invalid keys or may be the key-printer didn't work right, you have the customer send you the certificate card and you can refund him. It's by far the better system than using just a algorithm-based genuine check, especially for things that directly translate into money, like gift certificates.