How to allow visitors to send the same letter to their representatives.

This is somewhat hard to describe. I built a website for a group at http://swansweb.org. They want to be able to put a “Letter to your representative” online that visitors can then forward on to various public figures like senators and representatives. Ideally, they would like to have a popup listing each person’s senators by zip code in case they don’t know who their representatives are.

Anyone have any idea how to do this? Should I post this in a different forum?