What is the free mail service for Congress?

Answer 1

Legislation passed by the United States Congress on March 3, 1873 stated that: "the Postmaster General shall cause to be prepared a special stamp or stamped envelope, to be used only for official mail matter for each of the executive departments",[5] to this end, the official stamps were issued on July 1, 1873 for the Executive Branch and the departments of Agriculture, Interior, Justice, Navy, Post Office, Treasury, and War following the ending of the franking privilege though the stamps were only in use for seven years mainly due to the need for departmental postage accountability.[10] The Post Office also printed official stamped envelopes for their own use.[5] In March 1877 the stamps became unnecessary, because a congressional act permitted government departments to provide their own envelopes endorsed "Official Business"