As an incentive to try their online services the post office is offering five FREE premium 4x6 postcards mailed including postage when you register with USPS.com. You can even upload your own photos to be printed on the cards.

Use source code C0240 (both of those are zeros) when you register to get your free postcards.

Sign up as a new user at
www.usps.com/netpost/premiumpostcard.htm

(link has been corrected)

*Ok, if you didn't get any kind of message saying the source code was invalid, but you think you only have 1 free postcard, try this: Go into "my account", and it should tell you how many 4x6 (and 6x9) postcards you have available. You'll also see that information under "purchase history."