I don't know what some of these other people are saying, because I here a bunch of crap. Maybe supplements are completely different from where I'm at, but some of this sounds bat schtit. Where I'm at, any full-time driver is entitled to their 8HR Day. That includes cover drivers. Violating the contract for going home when management sends you home? How is that a violation? Hey, if you're meek, don't answer the phone.
Sending driver's home on the day they have an 8HR Day is just another way UPS attempts to put the screws to us. When you go home when they give you some lame-assed excuse why they can't cover it, you're just doing their dirty work by going home. If you didn't want to work, then just call in sick. When I used to get 8HR days, I sure as hail wasn't going to set my alarm, wake up, take a shower, dress up in the monkey suit, drive in rush hour traffic, wait around until start time, and then drive all of the way back home. It wasn't my problem that management couldn't plan for the 8HR Day they knew I had one day prior. If I came into work, I was working.
If you do this four or five times in a row, they will get the message...such and such needs to run this base car or that base car because we know he won't go home. And hold them to your 8HR Day, too. If it's too much work, call early and often. It's on them to fix the situation. Again, do it a few times and they WILL fix it.