i have shipped wheels before through the post office. i had to divide the set of 4 wheels up into 2 seperate boxes since the PO has a 70 pound limit on what they will ship. I cant remeber exactly but it was around $35 per pair to ship but they stayed here on the east coast. To protect them during shipping I had 2 old moving blankets that i cut up and wrapped around the faces of the wheels.
I prefer to use UPS for the bigger stuff though. My nearest UPS shipping location is about 30 minutes away so i usualyl just schedule a pick up at my house and its only about $3 or $4 more expensive to have them actually come to you and pick it up.
I would reccommend boxing 2 wheels togehter, then slap your package on a bathroom scale, and then get your lenghtxwidthxheight measurements, and you can caluclate the shipping costs on UPS.com
http://wwwapps.ups.com/calTimeCost?loc=en_US
I just call them. I like a live person 1-800-PICK-UPS
hope this helps