Shipping companies appear to have been caught off-guard in 2013, apologizing for not delivering packages that ought to have arrived by Christmas.NBC News' Gabe Gutierrez reports.

Two of the nation's biggest package shippers hit the road Thursday, scrambling to deliver boxes to angry customers who didn't receive the gifts they ordered in time for Christmas.

UPS and FedEx, under fire for failing to deliver Santa's packages in time for the holiday, blamed the delays on weeks of bad weather and higher demand from soaring online sales.

UPS didn't make pickups or deliveries on Christmas Day, but it brought in extra workers on Christmas night to the company's hub in Louisville, Ky., to sort packages for Thursday and Friday deliveries, a spokesperson for the company told The Associated Press. Some FedEx customers were able to pick up packages at local FedEx Express centers on Christmas Day.

Both companies, pummeled on social media by negative comments from furious, giftless customers, issued apologies.

"The volume of air packages in our system exceeded the capacity of our network. ... We apologize," UPS, which was plagued by more shipping problems than FedEx was, said in a statement. FedEx echoed that apology, calling the volume an "extraordinary event."

Sen. Richard Blumenthal, D-Conn., demanded that UPS provide refunds to customers across the U.S. who were quoted arrival dates before Christmas.

"I call on UPS to do the right thing and provide refunds to people whose Christmases were a little less cheery as a result of their late deliveries," he said.

In response, UPS said it would, indeed, issue refunds to customers who shipped by air or internationally. It said they could call 1-800-PICK-UPS (742-5877).

Tech expert Mario Armstrong presents some responses from users on social media after delays in holiday deliveries.

Besides poor weather in many parts of the country, the delays were blamed on systems overloaded by a shorter holiday shopping period this year due to Thanksgiving's having fallen so late in November and to more present-buying done online.

"Shippers this year really did not do a good job of anticipating the shift in holiday shopping," retail analyst Marshal Cohen said. "More consumers shopped last-minute. More retailers offered online deals and free shipping."

Online spending jumped 10 percent, to $42.8 billion, between Nov. 1 and Dec. 22, according to the online research firm comScore, and retailers expect overall holiday sales to be up nearly 4 percent, exceeding $600 million.

"We had a shortened holiday season in terms of shopping: six fewer days and a surge in online demand, particularly accelerated by a surge in mobile devices," said Mark Mahaney, an online retail analyst. "Mobile accounted to 40 percent of all Internet sales."

Neither UPS nor FedEx said how many of its packages were delayed, but they said it was a small percentage of the Christmas shipments.

Scott Fielder, a spokesman for FedEx Corp., said FedEx handled 275 million shipments between Thanksgiving and Christmas.

"Our 300,000 team members were focused on safely delivering our customers packages this holiday season and we are proud of their efforts," he said in a statement Thursday. "We operated with very high service levels — over 99 percent at FedEx Ground, for example — during our busiest time of the year."

The U.S. Postal Service said holiday volume jumped 19 percent over the same period last year, a higher-than-expected increase. Heavy volume was expected to continue throughout the week.

Jay Erikson, a customer who took to FedEx's Facebook page to express his frustration, was one of the unlucky ones who didn't get his parcels on time.

"If you can't keep a promise, then don't make it. Thanks for nothing, FedEx," he wrote.

Amazon.com, one of the nation's largest package shippers, cited UPS's "failure" in an email to customers on Christmas morning. The online retailer said it processed all orders on time and said it is compensating some customers with $20 gift cards and refunds on shipping charges.

UPS issued another apology Thursday afternoon.

"UPS apologizes to both shipping and receiving customers who may have delays. We had additional sorting operations at Worldport, the UPS international air hub in Louisville, Ky., on Christmas and are making deliveries and pickups today," it said.

The shipper also posted a service update on its website Thursday urging customers to track their packages via UPS.com and saying: "UPS experienced heavy holiday volume and is making every effort to get packages to their destination as quickly as possible. UPS has resumed normally scheduled service on December 26."

By Thursday evening, the company said it was well on its way to delivering the delayed packages.

"We anticipate having the backlog of packages cleared by tomorrow,” said UPS spokesman Natalie Black.

The UPS statements were released as complaints via social media continued to pile up.

"I will never use UPS again ... never!" Samuel Barnett wrote on the company's Facebook page after his product from Apple didn't arrive by Christmas, even though he ordered it second-day air more than a week earlier.

The reactions weren't all bad, though.

Heidi Grant, a North Carolina mother of three who didn't receive her gifts in time, told TODAY: "I guess the benefit is Christmas is going to last longer. [The kids are] going to have more gifts to open in a couple of days."

And on Twitter, others blamed procrastination of disappointed customers who had taken to using the hashtag "UPSFail" to complain about not getting their packages.

"People using #upsfail need to get a life," wrote @MrsKimSanders. "Order early next year!! Thank you UPS workers!!"

The last time a significant number of UPS packages were late for Christmas was in 2004, when an ice storm crippled its Louisville distribution center. Employees manually loaded packages for days that year, surprising customers with Christmas Day deliveries.

Tony Dokoupil, Gabe Gutierrez and Daniella Silva of NBC News contributed to this report.

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