There are 14 US Post Offices in Grant County serving a total of 79,796 residents equating to 5,700 residents per post office. It's estimated that approximately 127,363 packages are handled across the 14 county post offices annually. Grant County is a county located in the U.S. state of Washington. As of the 2010 census, the population was 89,120. The county seat is Ephrata, and the largest city is Moses Lake. The county was formed out of Douglas County on February 24, 1909 and is named for U.S. President Ulysses S. Grant. Grant County comprises the Moses Lake, WA Micropolitan Statistical Area."