It seems that for internet marketers, this may truly be the “most wonderful time of the year.” Last week marked the heaviest week of U.S. online shopping in history, with consumers spending more than $900 million on the web on four individual days, according to comScore data.

During the week ending December 17, online spending reached $5.15 billion. This demonstrates a 14 percent increase over the same week last year. While Green Monday (December 13) led the shopping week with $954 million in sales, Free Shipping Day (December 17) saw the greatest year-over-year gains.

Last Friday, consumers eager to take advantage of free shipping deals spent $942 million online. This represents a 61 percent increase over the same shopping day last year.

ComScore officials expect that marketers have witnessed the peak of online spending, and they believe the benefits of Free Shipping Day may explain the high volume of late-in-the-season sales.

“Free shipping has certainly become one of the prevalent themes of the 2010 holiday season,” said comScore chairman Gian Fulgoni. “Free Shipping Day also appears to have driven a sustained late-season response, with free shipping transactions accelerating in importance in 2010 whereas they actually began to decline during the same period in 2009.”

When Brafton first reported predictions for record-breaking online sales during the 2010 holiday season, we reported that one-third of consumers told comScore free shipping options would impact their purchase decisions. Marketers may take this year’s late-season sales success as an indication that free shipping promotions could boost sales year round.