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…