Unfortunately, right or wrong, the company can do say what ever they like in regards to how they disciple workers even if they only "believe" an employee is at fault. Your grievance will land like a smelly turd on the floor. Your contract gives you the right to grieve but that doesn't mean it will succeed if the company wants to give you a warning letter or charge you with an accident.
In my area warning letters never get heard at joint panels. So bet on a dead lock office trip with you and your BA with you walking out with a warning letter and an accident charged in your file. I know its unfair but I suggest you just move on without making a fuss about it. The only thing a grievance will do is make a already bad situation worse. In a few months, if your a good UPS Romper Room Do Bee, ask the center manager if they could withdraw the letters from your file.