There are 4 US Post Offices in Foster County serving a total of 3,452 residents equating to 863 residents per post office. It's estimated that approximately 5,510 packages are handled across the 4 county post offices annually. Foster County is a county located within the U.S. state of North Dakota. As of the 2010 census, the population was 3,343. Its county seat is Carrington. The county was founded in 1873 and later organized in 1883. It was named for George I. Foster, a pioneer and member of the Territorial legislature."