Stampr is a free letter mailing web service. Users simply submit a letter of five pages or less, and Stampr will print, fold, insert into an envelope, and send. Stampr prints in full color and works with any desktop application with a print button.
The Stampr API provides a RESTful interface for automating mailings. The supports the creation and control of letter batches as well as individual mailings. The API is designed to eventually support full customization, including options such as letter size, turnaround time, color values, and return envelopes.