There are 5 US Post Offices in Allendale County serving a total of 10,390 residents equating to 2,078 residents per post office. It's estimated that approximately 16,584 packages are handled across the 5 county post offices annually. Allendale County is a county located in the U.S. state of South Carolina. As of the 2010 census, the population was 10,419, making it the second-least populous county in South Carolina. Its county seat is Allendale."