Some packages were not delivered by Christmas Day because of shipping problems at UPS and Fedex Corp., officials at both companies said.
"UPS understands the importance of your holiday shipments. UPS is experiencing a heavy holiday volume and making every effort to get packages to their destination; however, the volume of air packages in our system exceeded the capacity of our network immediately preceding Christmas so some shipments were delayed," UPS said in a service update posted on its website.
UPS resumed regularly scheduled pickups and deliveries Thursday after taking the holiday off.
Fedex also apologized to customers for delays and said some customers were able to pickup some undelivered packages at FedEx Express centers.
"We give our apologies to customers," a Fedex customer support line supervisor told NBC News.
Internet retailer Amazon.com sent an apologetic email to customers who did not receive their packages in time for Christmas and offered $20 gift cards and refunds of shipping charges, Bloomberg News reported.
Online shopping increased 9 percent from Nov. 1 to Dec. 15 to $37.8 billion, online research firm comScore reported.