David Cook scans the barcode on a robotic bin after restocking it at the Amazon Fulfillment Center in Tracy, California, November 29, 2015. U.S. holiday shopping is on track for a modest 3.7 percent rise this year after strong turnout during the Thanksgiving and Black Friday weekend and thanks to strong online sales, the National Retail Federation said on Sunday after releasing an annual consumer poll. REUTERS/Fred Greaves