A 12 hour day would never happen in my center, even on December 23rd. They go up in arms at 11 hrs. Our days average 9 - 9.5. Over 10 once in a while.
One thing about this type of business is that it has to operate on a mandatory overtime model. One of the reasons being is that we are guarateed 8. So if they planned 8 and we finish in 7.5, UPS has a costly problem. The other main reason is that overtime for existing drivers does indeed cost less than more drivers (with health care and pension) and PC's on the road.
It's also the reality of an up and down business with differing volume each day. Can't over hire too much to cover the heaviest days because when the lightest days come, how many drivers will agree to stay home or how many will face layoffs?
Ironically, we have high union wages and bennies to thank for a lot of our overtime. If you took away the pension, you'd see more drivers/routes.