Last Tuesday marked another bittersweet milestone in the U.S. Postal Service’s seemingly endless downsizing campaign as almost 3,800 postmasters walked out the door. A few hundred more could follow them by the end of next month, according to USPS figures released today at FedLine’s request. The cause, of course, is the POStPlan, which calls for reducing customer service window hours at some 13,000 post offices nationwide to as little as two hours a day. As part of the same plan to save about a half-billion dollars annually, the Postal Service also intends to eliminate more than half of the 21,000…