Scales became fixtures in post offices in the mid-nineteenth century when postal officials tied postage rates to the weight of the mail. Before 1845 the Post Office Department calculated postage based on the number of pages in a letter and the distance it had to travel, leading to a very complicated cost structure. The Department changed the rate structure in 1845, basing postage on weight of the mail piece and whether it was to travel more or less than three hundred miles. In 1855 it cost three cents for a letter weighing half-ounce to travel up to 3,000 miles, which included most of the United States and its territories. The Act of March 3, 1863 (12 Stat. 704), eliminated postage differences based on distance.
Postal clerks used scales to weigh each letter and accurately calculate the postage. Many postal scales have the letter rates engraved directly on the scale. Until the early twentieth century most postal scales were small in both size and capacity because the Post Office Department had a package weight limit of four pounds. With the introduction of parcel post in 1913, post offices acquired larger scales that could accommodate packages up to seventy pounds.
Allison Marsh, National Postal Museum