I used to do it while I was in college years ago. Unless they have changed their standards, they only hire college students to do this type of work. I started out unloading semi-trailers. Most were completely packed with boxes from top to bottom and you had to take them off the pile and place them on the rollers and maintain a certain package per minute speed. It is very hard work. I would only work 3-4 hour shifts and I was completely exhausted afterwards. The shift began at 11pm and went to 2-3am.
After awhile, I moved to preload. I was in charge of taking packages off the belt according to zip code and street names and then I had to arrange them on the package cars in order of delivery. It took some time to get used to, but this was much better work. The package drivers were very nice- I guess they have to be, if they were rude to the people loading their trucks, it would be more work for them in the long run because they had to resort the packages!! This shift began at 4 am and went until 7-8am.
I finally had to quit because the drive was killing me to work 3-4 hours at a time. It took me 45 minutes each way and although the money and benefits were fantastic, I just couldn't handle it anymore.
Hope this helps and feel free to PM me with any questions.