I worked at UPS, and I worked at their HUB the biggest UPS place you can work at. It says up to 70 lbs... not all packages are 70 LBS. Most are very light. You can be skinny an work there. You wont gain much muscle its mostly about endurance. You lift the packages the whole time/no break cept for a 15 min one in the middle and there is a reason you only work 3-4 hrs a day.
The job is a bit draining, but pretty easy, especially after you have been there for a couple weeks. The hardest part about working at UPS was probably the weather heat/cold 90+ degrees means even more when you are in a non-ventilated trailer unloading a 40 foot trailer for 1 hour. Sometimes they also rush you to meet deadlines, but if you are slow they just replace you with a fast guy and put you on a easy trailer.
You can also be put on loading trailers, which is harder. Think of tetris and making a wall out of boxes and there you have it. Unloading is easier though.
And that $8.50/hr you are going to get is about $100-110 a week. They have union/medical fees and crap. Their pay is crap. I'd say.
Go work at a bank.