Card 13 recognizes the importance of US Mail is the earliest days of our republic. The first major postal law was passed by Congress in 1792. The law encouraged the creation and distribution of newspapers by allowing them to be mailed at a very low rate or, in some cases, free.
Title 39 of the US Code:
The Postal Service shall have as its basic function the obligation to provide postal services to bind the Nation together through the personal, educational, literary, and business correspondence of the people. It shall provide prompt, reliable, and efficient services to patrons in all areas and shall render postal services to all communities.