I just went to the post office to mail
After poking around that website I found that actually you have to play the game to become a "Jedi Shipping and Mailing Master" in order to be entered in the sweepstakes (It's not hard - they recirculate possible answers about USPS services, but each one is only correct once, and the Jedi trivia questions are easy). You're entered once for each level of the game you complete, it says. But then
In the meantime, you can vote for which star wars stamp will be printed on its own sheet in addition to the multi-sheet that will be printed (of the whole batch, I assume). You can vote once a day (per email address) for the next 15 days.
So go vote!
(me, I favor the Princes Leia/R2D2 design, the Millenium Falcon, and well, Boba Fett (sexy as always). Yoda and Darth Vader are in the lead. Which one(s) do you like?)