Americans waited until the last minute to buy holiday gifts, but retailers weren't prepared for the spike.

Heavy spending in the final days of the mostly lackluster season sent sales up 3.5 percent between Nov. 1 and Tuesday, according to MasterCard Advisors SpendingPulse, which tracks payments but doesn't give dollar figures.

Online shopping led the uptick, with spending up 10 percent to $38.91 billion between Nov. 2 and Sunday, research firm comScore said.

"We always have last-minute Charlies, but this year even people who normally complete shopping earlier completed shopping later," said Marshal Cohen, chief retail analyst at NPD Group.

But the late surge caught companies off guard. UPS and FedEx failed to deliver some packages by Christmas due to a combination of poor weather and overloaded systems.

Amazon is offering customers with delayed shipments a refund on their shipping charges and $20 toward a future purchase.Other retailers such as Macy's said they were still looking into the situation.

The last-minute surge this year solidifies the increasing popularity of online shopping, which accounts for about 10 percent of sales during the last three months of the year. But it also underscores the challenges that companies face delivering on the experience, particularly during the holiday season.

Analysts say FedEx and UPS typically work closely with big retailers, trying to get a sense of what the volume of packages will be during peak times like the holiday season. But this year, David Vernon, an analyst at Sanford C. Bernstein, said weather played a role. The early December ice storms in Dallas could have hurt operations, he said, and packages can start to accumulate. And that got compounded this year by a late surge in shipments, he said.

Another problem was the growing popularity of retailers offering free shipping. Amazon, for one, has a two-day free shipping offer that comes with its $79 annual Prime membership. Just in the third week of December, more than 1 million people signed up for the program, the company said.

"Frankly, the right hand wasn't talking to the left," said Forrester analyst Sucharita Mulpuru. "The marketing teams of a lot of web retailers (offering free shipping) were not talking to the operations and supply chain teams."

The delays could be a public relations headache, said Jeremy Robinson-Leon, CEO of Group Gordon, a corporate and crisis PR firm.

"It is a major problem for UPS and FedEx," he said. "The central pillar of their business is a perception of reliability with their customers."

Others say people will still buy online. "Consumers tend to have a short memory, especially if you fast forward to another year," said Andrew Lipsman, vice president of industry analysis for comScore.

Some shoppers are taking delays in stride. Traci Arbios of Clovis, Calif., did about 90 percent of her shopping online. Most items included free shipping and everything arrived on time except one.

"Everything arrived on time except this one item, it's not going to stop me from shopping online," she said.