According to this 10/11/31 account from Everyweek Magazine, the very first United States Post Office airmail flight took place on September 23, 1911 thanks to a Bleriot "Queen" with a 70 hp Gnome engine, numbered 13 on the tail and named Dragonfly by its owner-pilot, Earle Ovington.  He carried on his lap a sack containing 640 letters and 1,280 postcards during the flight from a small airport on Long Island, New York to the nearby town of Mineola.  The 10 mile trip was completed without mishap, at the end of which Earle tossed the sack from a height of 500 feet in the air onto a designated mark on the ground, thus inaugurating U.S. air mail service  -  the first non-military commercial aircraft application in the United States.

For more on this flight and the other pioneers of the United States air mail service visit this excellent site: http://www.airmailpioneers.org and view their version of Earle Ovington's historic adventure at http://www.airmailpioneers.org/history/milestone3.html