A week-and-a-half after their old contract officially ended, the U.S. Postal Service and its largest union will keep talking a while longer in hopes of agreeing to a new one. “We will be working late tonight and early tomorrow,” American Postal Workers Union President Cliff Guffey said in a web release this afternoon. The two sides will continue negotiations until they either reach an agreement or decide  that a deal is not possible, the release adds. In an e-mail, USPS spokesman Mark Saunders said only that “both parties have agreed to negotiate beyond today.” This is the third extension of…