There are 18 US Post Offices in Cass County serving a total of 117,377 residents equating to 6,521 residents per post office. It's estimated that approximately 187,347 packages are handled across the 18 county post offices annually. Cass County is a county located in the U.S. state of North Dakota. As of the 2010 census, the population was 149,778, and was estimated to be 171,512 in 2015. Cass County is the most populous county in North Dakota, accounting for nearly 23% of the state's population. Its county seat is Fargo, the most populous city in the state. The county was organized on October 27, 1873. Cass County is part of the Fargo, ND-MN Metropolitan Statistical Area."