With a surge in online shopping in the last couple of weeks, UPS is struggling to keep up, and delivery delays are expected. As the world’s largest package delivery company, UPS handles shipping for many of the biggest retailers, including Amazon, Walmart, and Macy’s.

UPS had been preparing for the peak holiday period all year, investing to build out and automate operations and capacity. They also added surcharges to deliveries during high-volume times to manage costs.

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Despite these preparations, UPS spokesman Steve Gaut told Bloomberg that online orders “overwhelmed expectations,” creating a “bubble” at some delivery centers. UPS says a majority of the peak season’s 750 million packages “will be delivered in accordance with the service commitments for the specified time-in-transit,” but UPS is adding one to two days of transit time to many of its deliveries.

UPS employees worked extra hours, including over the weekend to handle the heavy volume. Gaut said that UPS expects to have the issue resolved by midweek this week.

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The Wall Street Journal reports that 89.2% of parcels shipped through UPS Express last week were delivered on time, according to ShipMatrix. Meanwhile, FedEx Express saw 99.4% of its packages delivered the day they were due. The Journal notes that the U.S. Postal Service delivers more online orders than both UPS and FedEx and has expanded delivery hours to handle the increase in volume.