From Skeet Man and he is correct.
Print this label out and stick it or tape it to the box: http://www.type03.net/ammo/limitedquantity.pdf (you only need one of the squares, not all 4).
DO NOT use an ORM-D sticker or marking on your box. They are being phased out anyways in favor of the ones at the link I posted, and a LOT of ammo shipments are disappearing en route right now because of the high price of ammo, and a sticker stating "Small Arms Ammunition" on it SCREAMS "steal me".
Other than that, you can ship through a UPS or Fedex hub (rate both, but Fedex will probably be cheaper). You cannot ship though UPS Store or Fedex Kinkos or alike, it HAS to go to a hub. It CANNOT be shipped USPS.