The faces of William and Kate are everywhere: on china by Royal Doulton, on a music album, on an official coin from The Royal Mint (I haven't figured out if it's going to be legal tender or not), and now, on postage stamps.
You could have custom postage stamps, too, if you'd like! In fact, they might make really cool favors.
Just pick one of the vendors approved by the U.S. Postal Service, and send them your design.