The Act of Congress of March 3, 1845, established a relatively simple rate of postage: five cents for a half-ounce letter sent any distance less than 300 miles; ten cents for a half-ounce letter sent over 300 miles. The rate went into effect July 1, 1845, ending all the complicated rates that had preceded it. However, Congress failed to authorize the issuance of postage stamps to pre-pay these rates.
In response, Robert H. Morris, the postmaster of New York City, asked Postmaster General Cave Johnson to allow him to issue stamps for prepayment of letters going through his office. His request was approved, which quickly motivated a few postmasters in other U.S. cities and towns to issue their own stamps too.
These issues served the same purpose as the Penny Black—they allowed the postal patron the opportunity to purchase the stamps in advance and then attach them to the letter, which could be deposited at the post office day or night. These stamps are now known to collectors as the 'Postmaster's Provisionals'.
The era of the Postmasters' Provisionals ended when the Act of Congress of March 3, 1847, authorized the postmaster general to issue government stamps to satisfy the postal rates. The first government stamps, the 5-cent Benjamin Franklin and 10-cent George Washington, were distributed on July 1, 1847, making the Postmaster's Provisional stamps invalid.