Nearly 19,000 of the agency’s 644,000 workers have called in sick or are isolating because of the virus, according to the American Postal Workers Union. Meanwhile, packages have stacked up inside some postal facilities, leading employees to push them aside to create narrow walkways on shop floors.

Some processing plants are now refusing to accept new mail shipments. The backlogs are so pronounced that some managers have reached out to colleagues in hopes of diverting mail shipments to nearby facilities. But often, those places are full, too. Meanwhile, packages sit on trucks for days waiting for floor space to open so the loads can be sorted.

“[Customers] are screaming, ‘Where’s my package? Why did it go to Jacksonville, Fla., when it’s going to Miami?’ ” said Martin Ramirez, president of the APWU Local 170 in Ohio. “I can’t speak on that. I’ve never seen this before where these places are overflowing.”

The end result: Many families won’t see online orders arrive in time for Christmas.