"stalling food and medical supplies"

That's what the employer knowingly brought into risk when they decided to run understaffed to increase profits.

That's risk that the transportation provider brought. The medical company ordering supplies from some other company, who happens to use this transportation provider some of the time but not all the time, was not aware of and could not have reasonably foreseen this risk.

There are surely indemnity clauses all the way upstream. There are plenty of next day shipping options from 3 national carriers, plus same day options and just putting someone on a flight.

As Matt Bruenig has noted, the BATNA for many workers is dire poverty. Quitting isn't that easy.