The final week before Christmas—which retailers were looking toward to salvage a slow start to the season—included Dec. 18, known as Free Shipping Day, when hundreds of retailers offer no-cost standard delivery. Although a comScore spokesperson had previously noted that this is typically one of the heaviest online spending days of the year, it failed to crack the top 10 this year.
In total, the season has boasted a total of 10 billion-dollar spending days, compared with 12 last year.
The news is the latest blow to retailers, as online has been considered the bright spot in a gloomy holiday picture weighed down by deep discounts and low traffic.
(Read more: As the clock ticks, retail traffic continues slide)
Things appeared to be trending in the right direction at the start of the season, when Cyber Monday broke the record for the biggest online spending day ever in the U.S., bringing in sales of $1.735 billion. Then, comScore announced that the week of Dec. 9 marked the first time ever that each day of the workweek saw online sales top $1 billion.
And just this past weekend, carriers UPS and FedEx said some of their holiday deliveries were delayed, attributing the slowness to heavy volume. Amazon also had to limit the number of new members to its Prime service, in order to ensure on-time delivery for its current members.
(Read more: Some shoppers waiting for post-Christmas deals)
R.W. Baird analyst Colin Sebastian further noted that peak day orders at Amazon increased nearly 40 percent this holiday.
"We view Amazon's reporting of peak day orders as a positive indicator, given that the company had discontinued reporting this metric in 2011 and 2012," Sebastian said.