Shoppers nationwide are still waiting on some Christmas gifts to arrive one day after the big holiday. Now, big shipping companies like UPS and FedEx are apologizing for packages arriving late.

UPS said, "The volume of air packages in our system exceeded the capacity of our network. We apologize."

FedEx echoed that apology from its competitor, calling the high volume in shipments an "extraordinary event."

Hixson resident Darren Gallaher paid extra money to guarantee delivery by Christmas. He ordered a pair of Crocs shoes for his wife Friday. FedEx told him it would take 1-2 business days but now, almost a week later, he still hasn't seen the shoes.

"Sunday you get an email confirming it'll be delivered Monday and then it gives another time table of 8 p.m. on Christmas Eve," Gallaher said. "And still it's not here. You pay for the next day shipping and it's not here so it's a little frustrating."

The wife of a UPS driver wrote anonymously to Channel 3 Thursday, saying "He was out till 10:00 for the last three weeks, including Christmas Eve to try to deliver their packages.  He told me Christmas day, with tears rolling down his cheeks, how he hated not to get that package there knowing that it may be some child's gift."

Analysts say there could a number of reasons for the delayed shipments.

"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 accounted to 40 percent of all Internet sales," Online Retail Analyst Mark Mahaney told NBC News.

Amazon, which is one of UPS's biggest customers, said it's refunding shipping charges and giving customers $20 gift cards if their packages did not arrive in time for Christmas.

According to the Associated Press, neither company will say how many customers are affected but that it's a small percentage of overall holiday shipments.