Click caption to download a pdf with 2 postcards on an 8-1/2 x 11" page.
Print them on cover stock and cut in half for 2 postcards.
Write your message, name, return address, and senator or congressperson's
name and mailing address on the back (see sample below). 
Put a regular stamp (it's oversized) on the back upper right corner, and mail.

 This is how to write on the back.

This is how to write on the back.