Three cars and a belt split?...it sounds like you have made yourself to be a hero in UPS's eyes and now daily, you have impossible standards to attain.
Your best bet is to put in for a job transfer to another part of the bldg, start somewhere fresh, and work by the book from now on.
You need to document this time that the supervisors are working and file on it. If your coworkers don't like getting paid then you may as well. This is partly your coworker's fault for voluntarily going home when being asked to.
Your in a catch 22. In one hand you should file grievance on the supervisor for taking time. Their not allowed to send teamsters home then finish their jobs. Don't work off the clock ever even with the excuse of setting your area up. They try this stuff with me bringing me in ten minutes later then the preload to load my trucks. I am also pretty sure if you got more seniority and their bringing in lower people then you to start you can file on that to. Trust me you won't be the most popular person though if you speak up.