TBH, unless there's physical damage to the card, then the only other way it can possibly be damaged is by a static discharge. Unless the post office are testing EMP weapons in the broom cupboard I can't see any way a graphics card could be damaged this way unless it received a static shock whilst being handled at one end or the other.
As I've already told Wayne, if the PSU doesn't solve things then he's welcome to return the card to me for me to test. If it works fine for me then I'd return it at his cost, otherwise I'd refund it and try to claim off the post office, but frankly I doubt I'd get very far when there's no physical damage to the card.