Instructions:Register with Moonpig and get £5 off your order.
It looks like they might have a glitch as it worked twice for me on some cards.
I don't know when it expires.
Update: This seems to still be working for some people on standard sized £2.99 cards, but not the £5.99.
If all else fails... KKH576P or COSMOGIRL should give 50% off. Thanks goes to cwernham for the new codes.
This Code now gives 50% off all £2.99 cards