The union that Amazon.com Inc. workers recently voted to represent them has demanded the company start bargaining in early May and cease any changes to employment terms at their warehouse in the interim, according to a letter the group issued Saturday on Twitter. The Amazon Labor Union also demanded the retailer respect workers’ rights to union representation during disciplinary meetings, the letter said. Amazon did not immediately comment. On Friday, after some 55 percent of workers voted to make their warehouse in Staten Island, New York Amazon’s first unionized worksite in the United States, the company said it may file objections before the election result is certified. By Jeffrey Dastin