E-commerce sales will increase 15% during November and December over the same months last year, even though uncertainty about the economy may make consumers wait until later in the holiday shopping season to make purchases, says Jack Kleinhenz, chief economist with the National Retail Federation, a trade group.

In a conference call today with journalists and investors, Kleinhenz said that he expects overall retail spending to grow 2.3% during November and December, a significant increase from the 0.4% growth posted during the 2009 holiday season.

Consumers remain unsure of the economy, making them cautious about spending, Kleinhenz said, adding that he doesn’t see consumers emerging from this mindset for some time. Though personal income has jumped by 5.6% this year, consumer spending has increased only 0.4%, he said.

That means retailers have to work harder on promotional campaigns to get consumers to buy. “Retailers have to convince consumers that the recession is over and convince them to spend accordingly,” he said.

A survey conducted in September reports that 35.5% of consumers say they will spend less on gifts this year, said Pam Goodfellow, senior analyst at BIGresearch, during the call. Though that represents more than one-third of consumers, the figure is down several points from surveys conducted at the same time in 2009 and 2008, when the percentage of consumers saying they would spend less hovered near 40%.  “Shoppers are showing improvement from the last two selling seasons, but caution still lingers,” she said.