A postal treaty between the U.S. and Mexico was established in 1887. Similar to the postal agreement with Canada, mail was sent at the U.S. domestic rates instead of the international Univeral Postal Union (UPU) rates. The treaty remained in effect throughout the 2nd Bureau era of postage stamps. Shown below are a variety of uses to Mexico.
1st Class Mail
A postal treaty between the U.S. and Mexico provided mail services at a discounted rate. Four cents of postage paid twice the letter rate for up to two ounces for this cover sent from Michigan to Durango, Mexico.
First-class U.S. domestic letters were forwarded to Mexico at no additional charge due to the postal treaty.
Registered 1st Class Mail
A cover sent at the first-class 2¢ treaty rate with registry service (8¢) from Boulder, Colorado, to Mexico by way of the El Paso, Texas exchange office. A census maintained by the United States Stamp Society (USSS) lists 53 El Paso exchange labels recorded.
Twice the letter rate (4¢) for up to two ounces, plus an 8¢ fee for registry service on an advertising cover sent from Chicago to Mexico City. Handled in St Louis, Missouri, the cover received an exchange label before entering Mexico. There are 17 St Louis exchange labels recorded in the USSS census.
Postcard Rate
Instead of the UPU two-cents postcard rate, this card was sent for a penny due to the postal treaty.