Others have gotten write ups via next day air sent to home w no verbal or talking
Unfortunately I have no advice re: discipline other than for you all to fight it all the way, and do your own 'homework' as much as possible, especially if your union reps are not reliable ... and also to be prepared to face the consequences if you lose ... though you'd expect mgmt would think long and hard before canning drivers when the operation is already short-staffed.
If the company is serious enough about discipline that they'd let your local operation go down in flames, there may not be much you or the union can do. I wouldn't claim to know. I doubt they are,
but if they are and you've decided to fight: your best defense is surely based on safety. Last winter I piped up in a wild safety meeting about forced 6th punches. Several of my yellow-bellied brothers on the safety committee dared to argued that 'fatigue is not an issue if you follow the methods', as expected. It got wild when the division level safety guy (forget his title), the company man, spoke up to fully agree with me. He even admitted that the company's data is clear: working 6 days a week and up to 60 hrs substantially increases employees risks of both getting into accidents and getting injured.
If your jobs are really on the line, and you want to fight rather than cower ... why not blow it as wide open as you can? Get the media involved. Harp on the fact that UPS is putting not just its employees, but also the general public, at great risk because they won't properly staff their operation or else refuse excess volume they can't safely handle. And that this data-obsessed company is well aware that it is doing so. And not only that, but that rather than resolve the problem, company mgmt does its best to convince the employees that they are to blame for all accidents and injuries, not human beings but delivery machines.
Also: would be a great touch for y'all to call the center and demand an on-call pickup to RTS those disciplinary letters.