From a buddy's FB post:

If you're planning on sending mail to your congresspeople, I must pass along a great idea from a friend. Post cards get right through to the official's office; letter mail must be opened in a special process to check for anthrax, etc. My friend ordered a custom postcard from Vistaprint, with a patriotic illustration on the front and her name and town printed on the back at the top of the message section. That makes it easy for her to get her message to her elected representatives. The basic price is $10 for fifty of them--cheaper than buying them at your local convenience store!

I just checked Vistaprint. There are various nickel-and-dime fees for certain kinds of customization.

I am planning to do it, but if anyone can recommend a cheaper place to get postcards printed or something, I'm all ears. You can probably even do basic ones at Staples or something. I would LOVE to buy some that already have postage paid, instead of buying stamps, but that probably isn't realistic.