Americans spent nearly $6 billion online at retailers' Web sites last week, as the so-called Cyber Week followed in the record-selling footsteps of Cyber Monday and Black Friday, to continue what has so far been a strong holiday shopping season.

The $5.95 billion spent over the week beginning Cyber Monday and ending last Friday marked a 15 percent surge, according to analysis from online research firm comScore released Monday. Overall, online spending in the Nov. 1 to Dec. 2 period is also up 15 percent.

The fact is that the online channel is growing a lot faster rate, Andrew Lipsman, vice president for industry analysis, said in a phone interview Monday. You'd expect new records to be broken. We saw that with Black Friday and Cyber Monday, and we're seeing that again.

Cyber Monday, Lipsman said, was the best-selling online retail day in history. It was followed the next two days by two more billion-dollar spending days.

Two things drew consumers to keep spending, Lipsman said. First were the ever-extended deals, as the concept of a Cyber Week became reality for the first time for many online retailers. Second was the increased presence of free shipping, which enticed shoppers even further.

This is sort of a continuation that we've seen over the last several years, Lipsman said. The promotional periods are bleeding out on both ends. They're starting earlier, and they're being drawn out later.

We certainly saw that impact not only on Cyber Monday, but also on Tuesday and Wednesday.

Amazon, for one online retailer, was one to extend deals as the week progressed. The site contributed to the record online Black Friday sales, as well.

But free shipping saw the most drastic increase, as it jumped from about 52.2 percent in the week ending Dec. 4 last year to 63.2 percent in the week ending last Friday. That is further incentivizing consumers to shop online, Lipsman said. In the week ending Nov. 27, the percentage of transactions with free shipping reached its highest ever, at 64.4 percent.

Free-shipping percentages were significantly higher than they have ever been, Lipsman said. It's been an important trend on Cyber Monday for the last few years. And it's something that consumers have really learned to expect.

Indeed, according to another comScore survey, 78 percent of respondents said free shipping in online transactions was at least somewhat important. Thirty-six percent called it very important, meaning they would not make a purchase without it.

Lipsman also pointed out that despite the cost to retailers, free shipping generates higher average order values. And it's also created its own gimmicky marketing day - Free Shipping Day, which will fall on Dec. 16 this year.

Free shipping is one of the most important incentives that online retailers must provide during the holiday season to ensure that shoppers will convert into buyers, comScore chairman Gian Fulgoni said.

Consumers have come to expect free shipping during the holiday promotion periods, and retailers, in turn, have realized that they must offer this incentive if they want to maximize their share of consumer spending -- especially at the outset of the shopping season.

The analysis from comScore was more good news for retailers following similar record days on Cyber Monday and Black Friday. Online retail sales on Cyber Monday jumped 33 percent on a year-to-year basis, according to IBM Coremetrics.

And on Black Friday, IBM Coremetrics said online sales rose 24.3 percent, including a 59 percent leap in department-store online sales. In brick-and-mortar stores, shoppers spent a record $11.4 billion, according to Chicago-based research group ShopperTrak. The National Retail Federation said last week that over the four days beginning with Thanksgiving, sales rose 16.4 percent.

The record numbers led to bounties for retailers who were able to take advantage of shoppers' pent-up demand, such as Limited Brands, Macy's and Saks Inc.

Lipsman said the continual strong and record numbers bode well for the rest of the holiday season. He expects a bit of a lull this week in online shopping after the frenzy of Cyber Week.

But with the typically high-selling Free Shipping Day and Green Monday, which is a Monday with at least 10 days left before Christmas, more billion-dollar days should be on the way.

They all are marketing events, Lipsman said. But they tend to be pretty effective. I think it's pretty good marketing.