There’s growing concern about whether America’s shipping networks are prepared for growth in online shipping. An analysis by the software company ShipMatrix shows UPS on-time delivery performance last week falling to 91 percent, down from 97 percent during the same time last year.

“We have not seen error rates this high since 2013,” said Steve Osburn, who works with retail clients on supply chain at the consulting firm Kurt Salmon.

That may not sound too scary, until one remembers that 2013 was an absolute nightmare for carriers. Waves of packages didn’t arrive in time for Christmas and people were furious. As Christmas gets closer, the stakes get a lot higher.

UPS declined to respond to the ShipMatrix numbers, which were first reported in the Wall Street Journal, or to provide internal on-time performance numbers. But UPS is reassuring people that it has the situation under control.

“This is a typical phenomenon during peak, where the volume of shipments that come to our hubs varies from the forecasts that were given by our retailers,” said company spokesperson Steve Gaut.

The problem that challenges both online retailers and carriers is the increasingly unpredictable shopper. Now that people can buy anything, anywhere, on their phones, retailers and carriers are having a hard time figuring out their next moves.

Rob Martinez is CEO of Shipware, which helps clients manage shipping. His data show UPS performing better than other reports. But whatever’s going on out there, he has simple advice for consumers.

“Shop early,” he said.