There are 5 US Post Offices in Allen County serving a total of 12,751 residents equating to 2,550 residents per post office. It's estimated that approximately 20,352 packages are handled across the 5 county post offices annually. Allen County (county code AL) is a county located in southeast portion of the U.S. state of Kansas.It is 504 square miles, or 322,560 acres in size. As of the 2010 census, the population was 13,371. Its county seat and most populous city is Iola."