I worked for UPS as a part time package handler a few years back while I still in college. Warnning, if you are not in shape or can't stand the heat/cold/physical grunt work, don't bother to apply. It is very demanding on your body and you have to move the packages fast (every package in 2 to 3 seconds or less).
The benefits are great for part timers. You have tuitition reimbursement, healthcare, 401K, ect. even you only work part time.
You probably work the nighshift (7 to 11 p.m.) to unload trucks or graveyard (1 to 6 a.m.) to load the truck. Good luck and let us know how things turn out.
EDIT: if you get the job, remember to bring a big jug of cold ice water with you. You WILL need it. Wear some comfortable clothes and good comfortable boots. No need to be fancy, you are not in the office. Dress as contruction workers but without the hard hat.