There are 6 US Post Offices in Cass County serving a total of 14,404 residents equating to 2,401 residents per post office. It's estimated that approximately 22,990 packages are handled across the 6 county post offices annually. Cass County is a county located in the U.S. state of Illinois. As of the 2010 census, the population was 13,642. Its county seat is Virginia. It is the home of the Jim Edgar Panther Creek State Fish and Wildlife Area."