SAN RAMON, Calif.—Some who mailed holiday presents weeks early this year found they didn’t act early enough as Christmas arrived with their gifts stuck in transit. The U.S. Postal Service said on its website that it was “experiencing unprecedented volume increases and limited employee availability due to the impacts of COVID-19.” Austin Race of Grand Rapids, Michigan, placed an online order Nov. 30 for a collector’s model die-cast of a NASCAR racing car. It hadn’t reached his father after the Postal Service passed through his neighborhood Thursday night, even though he was notified Dec. 8 that it was shipped by two-day priority mail. His gift was in Opa-locka, Florida, the last time he checked the tracking number, about 750 miles (1,200 kilometers) south of where he ordered it in Mooresville, North Carolina. Race, 21, resigned himself to telling his father he will have to wait a little longer for his …