U.S. shoppers spent 15 percent more in online holiday buying compared to last year, after what may have been the busiest week of the season, said research firm comScore on Sunday.

Shoppers have spent $30.9 billion online from Nov. 1 through Dec. 16, up from $26.9 billion at the same point last year, said the Reston, Va., company, which tracks Web use.

Online sales surpassed $1 billion on four days last week. Total sales for the week climbed 15 percent to $6.31 billion compared to last year.

The five days that ended on Friday “will almost certainly be the heaviest week of the online holiday shopping season,” said comScore chairman Gian Fulgoni. Online spending will begin to slow as Christmas draws closer, he said.

But “Cyber Monday,” the Monday after Thanksgiving, is still the largest online shopping day ever, according to comScore. Sales for that day rose 22 percent from last year to $1.25 billion. Cyber Monday sales topped $1 billion for the first time last year.

The holiday shopping season can make up to 40 percent of retailers’ annual revenue. The online sales data point to Americans’ growing comfort with using their personal computers, tablets and smartphones to shop for the holidays.

Discounting and promotions have also boosted shopping this year. ComScore said on Sunday that shoppers have received free shipping on at least half of all their purchases in each week of this year’s holiday shopping season.