It’s been a record year for online holiday shopping in the U.S., and it’s still early December. We’ve looked at a lot of the stuff that was going on Cyber Monday and Black Friday already, but comScore has now put out a report indicating that the entire “Cyber Week” set a new weekly record.

$18.7 billion has already been spent online during the holiday season to date – a 15% increase compared to last year. There were three days last week that surpassed $1 billion in spending. Cyber Monday, of course, was the largest at $1.25 billion.

Tuesday hit $1.12 billion. Wednesday hit $1.03 billion. These three days were three of the four heaviest online spending days in history, comScore says. The other one was Cyber Monday last year.

“Cyber Monday kicked the week off with a bang as consumers opened their wallets to the tune of $1.25 billion, but it was only the beginning of a very strong week of online holiday spending,” said comScore chairman Gian Fulgoni. “Tuesday and Wednesday followed with billion dollar spending days, helping Cyber Week reach a record weekly total of nearly $6 billion in spending. As the deals from this week expire, it will be important to see the degree to which consumers return to the same retailers to continue their holiday shopping, thereby helping improve retailers’ profit margins, or if we experience a pullback in consumer spending – which has occurred in previous years – before promotional offers and spending intensity pick back up in earnest around mid-December.”

Here’s a year-to-year comparison:

Free shipping has clearly played a big role in all of this. Here’s another look at the growth of this:

Here’s a chart showing how important this is to people:

“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,” added Mr. Fulgoni. “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. In fact, more than three-quarters of consumers say that free shipping is important to them when making an online purchase, and nearly half say they will abandon their shopping cart at checkout if they find free shipping is not being offered.”

As the holidays get closer and closer, I’m guessing speedy delivery will continue to rise in importance to consumers. This is probably one reason that Google is looking at getting more involved in this area (to compete with Amazon).