The opening of Postal Service retail centers in dozens of Staples stores around the country is being met with threats of protests and boycotts by the agency's unions. The new outlets are staffed by Staples employees, not postal workers, and labor officials say that move replaces good-paying union jobs with low-wage, nonunion workers. "It's a direct assault on our jobs and on public postal services," said Mark Dimondstein, president of the 200,000-member American Postal Workers Union. The dispute comes as the financially struggling Postal Service continues to form partnerships with private companies, and looks to cut costs and boost revenues....