Final answer:
The addition to U.S. mail addresses on July 1, 1963, was the ZIP Codes, enhancing efficiency in mail sorting and delivery.
Step-by-step explanation:
The addition to U.S. mail addresses that occurred on July 1, 1963, was the introduction of ZIP Codes. The founding of the Post Office Department and the evolution of mail delivery, such as the establishment of rural free delivery in the early 20th century, are significant aspects of U.S. communication history. Prior to rural free delivery, individuals living in rural areas had to travel to the nearest town to collect their mail. With the introduction of ZIP Codes, mail delivery became more efficient, allowing for faster sorting and routing of mail across the country. Moreover, the advent of ZIP Codes represented a step towards modernizing the postal service and improving the delivery system that had begun with horseback carriers and evolved to include various forms of transportation, including the use of horses, wagons, and cars.