A young man from San Rafael crashed a car through the plate-glass window of the Stinson Beach Post Office on Saturday night after mistakenly driving in reverse while trying to exit the parking lot. The driver was sober, but according to Jesse Peri, chief of the Stinson Beach Fire Protection District, he did not have much experience behind the wheel. The tail end of the B.M.W. came to rest inside the area where mailboxes are located, separate from the entrance and retail counter. No one was injured and the post office opened on schedule Monday morning. The window is boarded up, and county building inspectors have examined the site to make sure the building remained structurally sound. “The car just went right through the glass and didn’t crash through the wall, luckily,” said Roosevelt Sargent, the Stinson Beach postmaster. “We never had to shut down, and today we’re completely open.”