While e-commerce sales will continue to grow over the next four years, its rate of growth will steadily drop as the online retailing market matures, according to a report today from eMarketer. E-commerce sales will grow 8.1% in 2015, down from 14.8% in 2010, the market research firm predicts.  EMarketer estimates that total U.S. e-commerce sales in 2015 will reach $269.8 billion.

EMarketer used available e-commerce estimates for 2009 and 2010 from the U.S. Department of Commerce as a starting point for its projections. The Commerce Department estimates e-commerce sales increased 14.8% to $165.4 billion in 2010 from $144.1 billion in 2009. EMarketer estimates e-commerce sales will increase 13.7% this year, with sales—excluding travel, digital downloads and event tickets—totaling $188.1 billion.

The research firm says sales growth will drop to 11.3% in 2012, 9.7% in 2013, 8.7% in 2014 and 8.1% in 2015. The firm projects the number of Americans buying online will increase 14.9% to 170.3 million in 2015 from 148.1 million this year, but says sales growth is more likely to come from existing online shoppers shifting their spending from stores to online rather than from new buyers.

The Commerce Department estimates 4.3% of all U.S. retail sales, excluding auto sales, fuel, travel, digital downloads and ticketing, took place online in 2010. EMarketer says that share will increase to 5.8% by 2015.

EMarketer principal analyst Jeffrey Grau says mobile commerce along with the spread of daily-deal sites such as Groupon and LivingSocial are two trends likely to boost e-commerce sales. “Both opportunities are expected to have strong sales growth over the next five years,” he says.