Nasty weather across the United States is causing headaches for shipping carriers and travelers alike.

FedEx Corp. on Tuesday posted a service alert on its website warning customers about delays due to inclement weather in the Southwestern and Midwestern United States. These delays come on the heels of pre-holiday storms that caused many last-minute online shopping orders to be late.

Chicago’s O’Hare International Airport, which operates more flights than any other in the United States, according to the Federal Aviation Administration, reported more than 265 flights had been canceled as of Tuesday morning, and some airlines were reporting delays of up to an hour.

FedEx didn’t specify how long the delays would be, saying on its site only that  “Although contingency plans are in place, some service delays and disruptions can be anticipated for inbound and outbound shipments in the Southwest and Midwest U.S. FedEx is committed to providing service to the best of our ability in areas that can be safely accessed.” A FedEx representative did not return an email seeking comment.

United Parcel Service Inc. also reported delays Tuesday morning, though not as widespread as FedEx. A spokeswoman told Internet Retailer via email, “Severe weather is impacting some areas of the upper peninsula of Michigan.” On its website, UPS posted a service alert informing customers that there will be no UPS Ground deliveries on Tuesday in 72 ZIP codes affected by Monday’s storms.

Meanwhile, the United States Postal Service reported on its website that post offices throughout Texas and Oklahoma were impacted by the storm that struck the Southwestern United States. Particularly hard hit was western Texas.

“U.S. Postal Service employees in facilities in the West Texas area affected by severe winter weather will be working around-the-clock to process and deliver mail for our customers,” the USPS said on its website. “The Postal Service will attempt deliveries where it is deemed safe for our employees.”