There are 22 US Post Offices in Charles County serving a total of 147,421 residents equating to 6,701 residents per post office. It's estimated that approximately 235,300 packages are handled across the 22 county post offices annually. Charles County is a county located in the southern central portion of the U.S. state of Maryland. As of the 2010 census, the population was 146,551. The county seat is La Plata. The county was named for Charles Calvert (1637\u20131715), third Baron Baltimore. Charles County is included in the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area."