There are 6 US Post Offices in Berkeley County serving a total of 65,432 residents equating to 10,905 residents per post office. It's estimated that approximately 104,437 packages are handled across the 6 county post offices annually. Berkeley County is a county located in the Eastern Panhandle region of the U.S. state of West Virginia. As of the 2010 census, the population was 104,169, making it the second-most populous county in West Virginia, behind Kanawha. Its county seat is Martinsburg. The county was founded in 1772. Berkeley County is included in the Hagerstown-Martinsburg, MD-WV Metropolitan Statistical Area."