Continuing on with the postage theme, here's a handy postmark for your letters. Just click through for the high resolution version.
Most printers should be able to handle printing this on an envelope or postcard with postage already affixed by using the normal envelope feed.
On a related note, if anyone with experience creating PDF's is reading this I could use some help. I think this graphic could be adapted for a PDF with data fields, making it infinitely customizable for whatever date a particular prop document needs. If you have any tips for doing that I'd appreciate any assistance you could provide. A postmark generator would be an extremely handy doodad.