Chalk Line is a printing and mailing source for missionaries and churches around the world. They print, fold, insert, stamp and mail your letters and postcards in a fraction of the time it would take you to do them. They can even help you design your newsletter, if that serves you.
Check out www.chalkline.org for pricing and details.
Note: Remember that one of your donors' favorite things is to have a personal hand-written note included with their newsletter. Consider printing locally the few that you want to personalize, and send the rest of your list to Chalkline!