According to the Wall Street Journal, the staffers included accountants and marketers, who suddenly found themselves hefting boxes. Such switches aren’t uncommon during the company’s hectic holiday season, but they’re usually voluntary and coordinated well in advance.

A UPS spokesman confirmed to the Journal that several hundred office employees have been called on to deliver packages. Some of them reportedly used personal vehicles. Most of the reassignments, according to the spokesman, are now wrapped up.

At least part of the problem was the tight labor market across the U.S., which made it harder for UPS to hire its usual bevy of seasonal workers. Another factor is online shopping, which has grown every year for more than a decade, and peaks sharply in the days before Christmas. This year, in certain locations, the number of packages exceeded even UPS’s projections.

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The unpredictability of that volume has been a challenge for UPS and other delivery services for years, and they’ve experimented with various solutions. Temporary staffing can increase throughout, but expensive overexpansion during the 2014 holiday season highlighted its downside risk.

On the other side of the equation, UPS has floated various approaches to raising prices during the holidays, in part to encourage customers to send packages earlier and spread out demand. This year, the shipper added peak surcharges, mostly under a dollar per package.

Those modest surcharges don’t appear to have done much to encourage customers to plan ahead this year. UPS could make them higher, but then the problem becomes competition — FedEx didn’t implement a holiday surcharge for most packages this year.

That leaves a nearly insoluble problem, as the delivery industry searches for ways to scale massively for a short period every year. Calling up the accountants doesn’t seem like a very sustainable solution.