>What exactly is the reason these shipping companies seem to go into a kind of hibernation over the weekend? Surely there are people out there who would be willing to work weekends (if the pay was right)? How much more would it cost them?

Supposedly preachers lobbied the government to shut down the USPS on Sunday because people were hanging out at the post office instead of going to church.

http://atheism.about.com/b/2006/02/19/sunday-mail-service-in...