I've never worked at UPS, but I started as a package handler at Ground. I imagine it's very similar depending on the sort you work. Move and scan boxes. If you are good, you can get as many hours as your manager can find for you and still classify you as part time. If you don't care, you can do a crappy job, they will get you off the clock as quickly as they can but it's pretty hard to get fired. If you show up everyday you'll keep your job.
I did it all through college, it's fine. Get paid to work out, no stress. I think they still have an extra $1/hr tuition reimbursement which is nice.