I've been a part timer for almost 10 years. Not by choice though, it really does take at least 13 years to get a FT driving position, and 16 years to get a combo. Luckily most of that time it's been in small sort. Most part timers want two basic things: more hours and bigger raises.
A guaranteed 25 hour work week for the more senior PTs wouldn't be bad. That's what most of us get anyways, it's just adding an extra bit of job security.
Also an outlined way to double shift. Make it sort of a bidding list that the sups would have a abide by. I know there are a lot of people who have other full times jobs or own a business that also work at UPS. Those people always want to get cut early, so why not give the hours to the people who want/need the hours? I wouldn't mind staying/waiting for the next shift, if there's a good chance someone will want to get cut and I can take their spot.