There are 6 US Post Offices in Traill County serving a total of 6,576 residents equating to 1,096 residents per post office. It's estimated that approximately 10,496 packages are handled across the 6 county post offices annually. Traill County is a county located in the U.S. state of North Dakota. As of the 2010 census listed the population at 8,121. Its county seat is Hillsboro, and its largest city is Mayville. The county was formed in 1875."