There are 8 US Post Offices in Cass County serving a total of 14,542 residents equating to 1,818 residents per post office. It's estimated that approximately 23,211 packages are handled across the 8 county post offices annually. Cass County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 13,956. Its county seat is Atlantic. It was named to honor Lewis Cass, who was the 1848 Democratic nominee for President."