The document is the U.S. Constitution; it established post offices. Article I Section 8 says "The Congress shall have Power To [...] establish Post Offices and Post Roads".
This power was given to Congress when in<em> June 1788</em> the ninth state ratified the Constitution. The following year president Washington appointed Samuel Osgood as the first Postmaster General, and 75 Post Offices and 2,400 miles of post roads were created. Almost four million residents benefited from this arrangement.
