U.S. online spending should increase 15 percent this holiday season, outpacing even last year's torrid growth, as shoppers are enticed by more retailers offering free or low-cost shipping, analytics firm comScore said.

ComScore estimated on Wednesday that e-commerce sales for the November and December period would reach $37.6 billion this year, after getting off to a fast start: the firm said on Tuesday that online sales were up 14 percent in the first 20 days of November.

The bullishness over e-commerce sales comes as retailers both on and offline prepare for the key U.S. Thanksgiving holiday weekend when people head to stores -- or to their computers -- in search of bargains.

For the entire 2010 holiday season, the increase in online sales was 12 percent.

A comScore survey of 1,000 shoppers earlier this week found that free shipping was a decisive factor for 76 percent of people in opting to buy an item on line.

Traditional retailers have made shipping cheaper, and in many cases, free of charge altogether this year to compete with one another and with online retailer Amazon.com Inc's Prime shipping program.

For instance, Macy's Inc lowered its free shipping threshold on all online purchases made next Monday. Nordstrom Inc began offering free shipping in August.