A week after the post office in downtown Winthrop was destroyed by fire, the U.S. Postal Service has begun making plans to replace it. The interior of the post office was gutted and its metal roof collapsed after a fire started in its attic last Tuesday morning. Its brick walls are still standing, but they will need to be knocked down, said Steve Doherty, a spokesman for the Postal Service. “They’re going to need to demolish what’s left of the building,” he said.