A year ago, Amazon.com workers like 34-year-old Rejinaldo Rosales hiked miles of aisles each shift to "pick" each item a customer ordered and prepare it for shipping. Now the e-commerce giant boasts that it has boosted efficiency - and given workers' legs a break - by deploying more than 15,000 wheeled robots to crisscross the floors of its biggest warehouses and deliver stacks of toys, books and other products to employees.