Web page scratch off games

[KAW]

I'd like to start a promotion using scratch off games to give buyers discounts on products, shipping or free gifts. I've seen them on other sites (PCH and gambling) and was wondering how it is done. I'd want a random coupon generated under the scratch off. Any ideas?

[Oyaru]

I would say it would be made in flash and if you have a store online as well, the coupon would be linked to the mySQL database(or the equivilent)

I dunno though.