Holiday shoppers in the U.S. spent 15 percent more on online purchases compared to the same time period last year, according to ComScore, a company that tracks Web use: And more Americans are using their personal computers as well as mobile devices to cross items off their holiday shopping lists.

From Nov. 1 through Dec. 16, shoppers doled out $30.9 billion on online purchases, up from $26.9 billion during that same time period in 2010.

Last week, total online sales reached beyond $1 billion on four separate days. But Cyber Monday was still the largest online shopping day of the season. On that day, the Monday after Thanksgiving, online sales rose 22 percent from last year to hit $1.25 billion.

Also of note this year is that Black Friday (the day after Thanksgiving), which is traditionally when shoppers hit retailers' brick-and-mortar outlets, saw the greatest percent change in the amount spent by shoppers online when compared to last year: Shoppers spent a total of $816 million buying items online. Black Friday was one of six specific online shopping days that ComScore compared to last year (as opposed to longer time periods), other days included Thanksgiving, Cyber Monday, and Free Shipping Day.

As the holiday shopping season winds down, you can find lots of tips and advice at our holiday-shopping hub.

Heaviest Week in U.S. Online Holiday Shopping History Pushes Season-to-Date Total to $32 Billion, Up 15 Percent vs. Year Ago [ComScore]