Paul Sakuma / AP, file

According to the digital marketing analysis firm comScore, U.S. shoppers already have spent a whopping $18.7 billion online during this year's holiday season.

Online shopaholics, get your mouses ready.

All signs indicate that the deluge of e-commerce deals offered during Thanksgiving week will continue.

According to the digital marketing analysis firm comScore, U.S. shoppers have already spent a whopping $18.7 billion online during this year’s holiday season, starting in November -- a sum that represents a 15 percent increase in spending over the same period last year.

And while big box retailers touted Black Friday deals for the weeks leading up to the Thanksgiving holiday, e-commerce has flourished throughout the entire week of Thanksgiving.

For example, comScore reports that consumers spent over $1 billion on three individual days last week. They shelled out a total of $1.25 billion online on Black Friday, a 22 percent increase from the $1.028 billion spent online on Black Friday last year. And shoppers spent $1.12 billion online on Tuesday, Nov. 29, and $1.03 billion on Wednesday, Nov. 30.

ComScore attributes the online shopping boom, which it says has reached “record levels,” to the preponderance of free shipping offers from online retailers. In fact, more than half of all holiday shopping transactions performed online this holiday season have included free shipping. During the week of Thanksgiving, 64.4 percent of all online transactions included free shipping, which is a 10 percent increase in its availability during the same week last 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,” said comScore chairman Gian Fulgoni in a statement.

“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,” Fulgoni said.

In comScore’s annual survey of consumers who shop online for the holidays, 36 percent reported that free shipping was “very important,” saying that they wouldn’t make a purchase without it. And in a different survey, nearly half the shopperspolled said they would abandon their online transaction if free shipping wasn’t offered after they’d put items in their online carts.

But with many online detailers continuing to offer slashed prices and special deals through the month of December, perhaps shoppers won’t need to wait until Free Shipping Day, which is on December 16, to do the bulk of their Christmas shopping.