Until the first U.S. stamp was issued in 1847, postage was paid by the recipient of mail. In 1851, prepayment of postage was made mandatory. The California Gold Rush of 1848 made immediate the need for transcontinental mail service. Steamships provided service by way of Panama. Improved overland stagecoach routes reduced transcontinental transit to 20 days.
Although a private venture, the Pony Express offered a horse courier service that reduced transit between St. Joseph, MO, and San Francisco, CA, to as little as six days. The transcontinental railroad allowed a seven-day mail between New York, NY, and San Francisco, CA. The Railway Mail Service, established in 1863, remained the greatest postal innovation until just after World War II. Rural Free Delivery went nationwide in 1896.
By 1915, automobiles in the RFD service allowed extensions of the rural routes. By 1920, there were 43,445 RFD routes. A national parcel post system began in 1913. Other innovations were machines to apply cancellations, mechanical sorting devices and underground pneumatic tubes. By 1924, the New York-San Francisco air mail route was flown westbound regularly in 34 hours.
Expansion of the air mail service was stepped up following World War II. More recent innovations are linked to even greater levels of automation, which included the institution of a national postal code system--ZIP--in 1963.