With just four days until Christmas, online retailers are about to wrap up another strong shopping season, a couple of recent studies indicate.

Boosted by a surge in sales last week, consumer spending online has grown 26% in the holiday season through Friday, according to comScore Networks. That echoes a separate survey by Nielsen NetRatings that, in conjunction with Goldman Sachs and Harris Interactive, found that online sales are up 28% over last year.

A number of factors have contributed to the growth, said Dan Hess, senior vice president of comScore: Consumers who first connected to the Internet in the last couple of years have steadily grown more comfortable shopping online. A growing number of retailers now offer customers the ability to buy online and pick up in their stores, making it easier for them to acquire last-minute gifts. And many online stores sent reminders to their customers last week that the deadline to order in time for Christmas was approaching.

"It's been a good, solid season, as we expected," Hess said. "In many ways,

this year's sales growth is the culmination of a great deal of systems and marketing improvements and learning and work that's been done over past couple years."

Despite the good holiday cheer, e-commerce stock were largely mixed on Tuesday.

Amazon.com

(AMZN) - Get Report

,

GSI Commerce

(GSIC)

and

Blue Nile

(NILE)

were up about 1%, 4% and 2%, respectively, in recent trading. But

eBay

(EBAY) - Get Report

,

Overstock.com

(OSTK) - Get Report

,

eCost.com

(ECST)

and

Shopping.com

(SHOP) - Get Report

were all trading off.

Online retail's apparent success stands in contrast to the

spotty picture offline. While some reports indicate that brick-and-mortar retailers are seeing a strong holiday season, others indicate that sales are coming in below expectations. Among those store chains seeing a disappointing holiday season in the offline world:

Wal-Mart

(WMT) - Get Report

, the world's largest retailer, which expects its same-store sales to grow just 1% to 3% this month.

In contrast, online sales appear to be torrid. Last week, for instance, online sales grew 49% over the same period last year, according to comScore. "The season's been a bit of a roller coaster," Hess said. "Depending on the day and week, we've bounced between the high- and low-end of our forecasted range."

(ComScore tracks actual transactions made by some 2 million consumers at North America-based online stores. Its data exclude sales of travel services, purchases made at auction and large corporate purchases. In contrast, Nielsen NetRatings and its partners regular survey more than 1,000 U.S. consumers about their online shopping behavior. Their data exclude online travel purchases, but include sales made at auction sites.)

As with years past, online sales peaked a little more than two weeks before Christmas, this year on Dec. 7, Hess said. What's different this year, though, is that sales didn't tail off so dramatically after that peak as they have in past year, he said.

Online shopping is seeing "a change in the shape of the sales curve," Hess said.

Sheer convenience is helping to convince consumers to spend more online, said Heather Dougherty, a senior research analyst at Nielsen NetRatings. With a majority of Americans having broadband Internet access at home now, consumers can shop after hours, and not just using their fast connections at work, she said.

Among the categories of products that seeing the fastest sales growth are music, DVD and videos and jewelry, according to the Nielsen NetRatings survey. Consumer spending on all three categories has grown more than 30% this year.

Jewelry sales, in particular, have been impressive, said Dougherty. In the six weeks through Dec. 12, consumers had spent nearly $1 billion on diamond rings, watches and related products, according to Nielsen NetRatings' data.

In the early days of e-commerce, a number of companies tried to convince customers to buy jewelry online, with little success, Dougherty noted. But consumers finally seem to be growing comfortable enough with online shopping to make such sizable purchases, she said.

"That's a category that seems to slowly be coming into its own," she said.