UPS says some package deliveries are being delayed because of an unexpected flood of orders from online shoppers.
Responding to complaints on social media, the company says it is working to fix the problem.
UPS spokesman Steve Gaut says the delays of one or two days were caused by an unexpectedly big "bubble" in online orders the week after Thanksgiving.
United Parcel Service Co. expects to be back to normal operations by the end of Wednesday. It plans to hire about 95,000 seasonal workers and use more automation to meet the crush of shipments.
The Atlanta company is sticking with its forecast of holiday shipments of more than 750 million package deliveries between Thanksgiving and Dec. 31, a 5 percent increase over last year.