Retailers have reason to be glum about the coming Christmas sales season. The National Retail Federation estimates sales will grow just 3.6% this year, higher than last year’s 3.2% increase and above the 10-year average of 2.5%, but down from 4.1% two years ago. It’s not helpful either that the NRF had forecast Christmas sales would rise 3.7% in 2015; consumers may disappoint them once again.