A woman is in custody for the fatal stabbing of a postal worker following a dispute over sandwiches at a Manhattan deli on Thursday, law enforcement sources told the Daily News. The 36-year-old mailman was on duty for the U.S. Postal Service when he stopped at Joe’s Grocery on Lenox Road near W. 118th St. to order a sandwich around 2:30 p.m., cops said. When the postal worker placed his order, a woman there became enraged, claiming he had cut her in line and the two began shouting at each other, a witness told The News. “It was over a...