How do I pack a road wheel for shipping?
Wheels generally come in a cardboard box with plastic caps over the hub ends to protect them and a bit of other padding (for skewers and the like) added in.  Your LBS may have a few of these laying around for whatever size wheel you are sending back.  They may even be willing to do the package job for you and save you a bit of time for a reasonable amount of money.  If you just want the materials, I have known some shops to simply give them away (especially to known customers).

The stresses a wheel endures in a cardboard box during shipping are less than what they would endure properly installed on a bike.  Any damage more than that would completely destroy the cardboard box and be evident as a shipping mishap.  I recommend insurance from your shipper to protect against such mishaps.