The Postal Service is finally moving ahead with its plan to cut hours at post offices. It will begin notifying communities after Labor Day that their local post offices may be open for just two or four hours a day. It is going to send surveys to residents and hold public meetings. The first 4,500 post offices could cut their hours by November. When fully implemented, the plan could save the agency $500 million. It cleared a hurdle last week when it got the green light from regulators.