And don't ever believe what a supervisor tells you.
UPS management is probably the most deceptive group of individuals that I have ever worked for and it's more than likely a way for them to 'deal' with the union. Although, they could probably accomplish so much more in by being fair and honest, they choose to take the complete opposite approach. Instead,
when people like yourself find out that they were screwed over, which most of the time
we do, that's what starts the grievance procedures. It didn't take too many times of being wronged for me to file on everything I saw. I'd file on a supervisor looking at me the wrong way if I could.
But yes, you want to file a grievance. If this is the first time in doing so, putting the pen to paper will be one of the harder things you've done in your career here. If you don't file, you will undoubtedly regret it.