After losing his job with Peet’s Coffee & Tea, Bill Hackel landed several part-time positions, but none that elicited quite the enthusiasm at home as his current gig: riding shotgun as a holiday “jumper” to deliver packages for United Parcel Service Inc.

Says 48-year-old Mr. Hackel of San Francisco: “I came home with the uniform, and my 12-year-old son said, ‘Dad, that’s pretty neat. Do you get to drive around in the big brown truck?’ ”

The weeks between Thanksgiving and Christmas give Mr. Hackel is among the 37,500 people who get a chance to suit up and ride along as temporary UPS driver helpers, a.k.a. jumpers. (The “jumper” name comes from the tiny fold-up passenger jump seat that helpers sit on in trucks.)

This year, Dec. 22 is expected to be the busiest day of the year for UPS, with an estimated 24 million packages delivered world-wide—almost 60% higher than the company’s normal daily volume.

All the big delivery outfits face the holiday crush. FedEx Corp.’s operating companies have largely depended this year on drivers working more expeditiously and longer hours to move more than 223 million shipments world-wide between Thanksgiving and Christmas. Same with the U.S. Postal Service, which has a hiring freeze and expects to deliver 152 million packages in the same period, up about 5% from last year.

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