There are 6 US Post Offices in Hamlin County serving a total of 5,975 residents equating to 996 residents per post office. It's estimated that approximately 9,537 packages are handled across the 6 county post offices annually. Hamlin County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 5,903. Its county seat is Hayti. The county was created in 1873 and organized in 1878. It was named in honor of Hannibal Hamlin, Lincoln's first vice-president."