[SOUTHFIELD, MICHIGAN] Amazon.com, the largest online retailer, offered US$20 gift cards and refunds on shipping charges after United Parcel Service Inc said overwhelming volume left it unable to deliver some packages by Christmas.

Amazon cited failures in UPS's transportation network in messages to customers, saying its own fulfilment centres processed customers' orders in time for holiday delivery.

"We are reviewing the performance of the delivery carriers," Mary Osako, an Amazon spokeswoman, said in an e-mail.