Amazon.com is offering US$20 gift cards and refunds on shipping charges to customers who did not get their Christmas orders on time, IDG News Service reports.

The online retailer is blaming the delivery companies for the widespread delays.

"Amazon fulfillment centers processed and tendered customer orders to delivery carriers on time for holiday delivery," Mary Osako, a company spokeswoman, said in an emailed statement Thursday, Dec 26. "We are reviewing the performance of the delivery carriers."

The company has refunded any shipping charges associated with impacted shipments in addition to offering the gift card. It did not disclose how many customers had been affected by the delays.

The delays highlight that even as online retail sales are picking up, the physical infrastructure for deliveries may often fall short of the requirements. United Parcel Service of America said on its website on Thursday that it "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 Thursday, it added.

"UPS stole XMAS this year...waiting for a bunch of packages," wrote an user on Facebook. Competitor FedEx was also criticized online.

"Our 300,000 team members delivered outstanding service during this holiday season, and we experienced no major service disruptions in the week before Christmas despite heavy volume," FedEx said in a statement Thursday. "Every single package is important to us, and we will continue to work directly with customers to address any isolated incidents."

U.S. shoppers using desktops spent about $42.8 billion between Nov 1 and Dec 22, up from $38.9 billion in 2012, according to digital analytics firm comScore. The figures do not include purchases through mobile devices.

The complaints about late deliveries came as Amazon.com reported that the entire 2013 holiday season was the best ever for the retailer, with more than 36.8 million items ordered worldwide on Cyber Monday, the Monday after Thanksgiving in the U.S., which was a record-breaking 426 items per second.

Some other retailers also reported delays.