Article I section 8 of the Constitution states "The Congress shall have Power ... To establish Post Offices and post Roads". The reason the founders gave the Federal government this power was because free and easy dissemination of information and ideas is necessary for the health of a democracy. They debated whether Newspapers should be mailed for free or for a nominal fee.
In modern society, the Internet serves this purpose. It allows for widespread transmission of information to the world from anyone. High speed internet access is essential for the survival of a modern democracy, yet access is controlled by regional monopolies or non-existent in rural communities. The USPS could establish a new source of revenue by investing in nationwide broadband with equal access for all Americans.