Brownslave688
You want a toe? I can get you a toe.
A strong core/with proper package handling is key. It is hard work, but getting a strong core is priceless. A strong core also minimizes the chances of improperly handling packages. Also strengthen your legs, and routinely stretch out your calves and hams. Limber limbs minimize undue stress on your back. Once you get these down you will work back injury free.
I would like to point out that cold stretching before a workout/UPS shift has been proven to cause more injuries than it prevents. Stretching is good-- if it's before a workout, though, it should be done after a warm-up but before the actual exercise. The optimal time to stretch would be after strenuous activity, not before.
That's why HOT yoga is all the craze now.