In cities such as San Francisco and New York, letter carriers have been showing up on people’s doorsteps as early as 3 a.m. bearing unusual cargo: milk, eggs, and other perishable items. The U.S. Postal Service has been delivering groceries to customers of Amazon.com in selected areas since October 2014. “It’s just leveraging our infrastructure,” says Megan Brennan, who was sworn in as America’s 74th postmaster general in March, becoming the first woman to hold the job in the institution’s 240-year history.