There are 12 US Post Offices in Clayton County serving a total of 18,110 residents equating to 1,509 residents per post office. It's estimated that approximately 28,906 packages are handled across the 12 county post offices annually. Clayton County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 18,129. Its county seat is Elkader. The county was established in 1837 and was named in honor of John M. Clayton, United States Senator from Delaware and later Secretary of State under President Zachary Taylor."