Amazon says it will give $US20 gift cards and pay shipping costs for customers affected by problems at UPS and FedEx that delayed some Christmas package deliveries.

The Amazon pledge came after UPS in particular came under fire for late packages despite vows from retailers to meet a December 25 deadline.

Some customers took to Twitter to voice their displeasure, likening one or both delivery giants to the “Grinch who stole Christmas”.

Amazon pointed the finger squarely at the delivery companies. The online retail giant did not give estimates for the number of affected shoppers.

“Amazon fulfilment centres processed and tendered customer orders to delivery carriers on time for holiday delivery,” said Amazon spokeswoman Mary Osako. “We are reviewing the performance of the delivery carriers.”

UPS spokeswoman Natalie Black said the problems stemmed from a “perfect storm” caused by an unexpected jump in Christmas shopping in the last two weeks, a compressed holiday shopping season in 2013 due to the late date of Thanksgiving and some severe weather that halted some deliveries.

“The result is that some packages that were set to be delivered on Christmas Eve were delayed,” Black said.

A FedEx spokesperson also reported a “surge” in volume, but said the rise was typical.

“We had minimal service disruptions despite the increase in volumes, and are working directly with customers who may have experienced any delays,” said the FedEx spokesperson.

Investors shrugged off the news. UPS gained 0.2 per cent, while FedEx rose 0.9 per cent.

The delivery woes suggested the retail sector is still adjusting to shifting customer behaviour with the rise of online shopping.

More retailers have promised to execute Christmas-deadline deliveries ordered later and later in the season.

“There will be a few glitches in the next few years,” predicted Chris Christopher, director for consumer economics at IHS Global Insight. “Whenever you’re growing at double digits like this, there’s bound to be this kind of thing happening.”

Christopher expects online sales gains of 13.5 per cent for the 2013 holiday season, compared with an increase of just 3.2 per cent for the overall retail market.

Amazon characterised its overall holiday shopping season as the “best ever” in the company’s history.