I have been searching for an answer to this for a little while this evening on various places on the internet. Everyone seems to have an answer but not very many people can back it up. Since it seems to me the answer should be very simple to find and wasn't I thought I would post my findings.
I found this link to the US Postal Service.
http://pe.usps.com/text/pub52/pub52c3_016.htm
"Small Arms Ammunition. Ammunition is classified as a Division 1.1, 1.2, 1.3, or 1.4 explosive, depending on the degree of hazard. Ammunition that is regulated as a Class 1 explosive and designed to be fired from a pistol, revolver, rifle, or shotgun, as well as associated primers and blank cartridges (including those designed for tools) and propellant powder for use in any firearm, is prohibited from mailing."
"Associated primers" seems to be the kicker here. Spent casings in my book qualify as scrap metal, brass tubing, etc.
UPS seems to be the way to go, as I have found this information in several places for shipping ammo.
http://cartridgecollectors.org/shipping.htm
It looks like you just have to declare the package as ammo (I don't know why primed cases would be any different) by putting a label as such on the package. There is no haz-mat charge. I'll try to find out, when I talk to them tomorrow, where a link to the info is.
Has anyone else found anything different?