Companies pay their hourlys to do a job in the time they are scheduled to work. As soon as you are past that scheduled time and off the clock UPS is no longer paying you and no longer has authority over you, other then telling you what you "can't" do with or on company property. Its a nice beneift of knowing that when a certain time is reached you are always free to leave. Salaried employees unfortunately don't have this benefit because they have to work as long as the company wants them to. If a supervisor asked you to do something off the clock then you absolutely do not have to do it, but I think its a good idea to consider the circumstances. If its something real simple that is going to be a pain in the butt for anyone else to do then you should consider doing it. If its something a bit complex and there are other employees around that can easily do it and are still on the clock then I would say reject the request if you want and don't feel bad about it. If I feel its a reasonable request then I atleast give a serious thought.
There are 2 rivaling mentalities, between the employees and management, that have really disgusted me in the 8 years I have preloaded at UPS. One being employees who will refuse to do anything if there not getting paid for it, and the other being the company nickel and diming employees with forcing them to punch out right when that 3.5 hours is up or shortly before for employees that don't care about getting the 3.5, or telling employees they must punch out before they wash their hands, or not even changing the start time even though there are 5,000 more packages, or trying to save a couple minutes off the pick workers time by sending them home a few minutes early and hope that no missorts gets that far into the system, etc. etc.
