From what I understand you are not allowed to ship ammunition through the US Postal Service and that you could be breaking the law if you do so. Here is information from the USPS site (note the 'is prohibited from mailing' at the end):
"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."
Here is the USPS link:
http://pe.usps.com/text/pub52/pub52c3_016.htm
My understanding is that only UPS accepts freight of ammunition as non-hazmat material and then only through UPS Ground and the package must be marked as ORM-D - Cartridges, Small Arms.
FedEx will accept ammunition but only as hazardous material (which would incur a hazmat surcharge).
To make sure you are in compliance with shipping restrictions, UPS Ground would be the way to go for shipping ammunition. All you have to do is to either write 'ORM-D' on the box, or google and print a free 'ORM-D' label, and affix it to your package.