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When you can, ship US Postal service. Their prices are much better than those of UPS and Fedex. I'm not sure what the size restrictions are, though.
USPS is one good option. Another is DHL. Apple computer uses them for their warranty pickups. I've had good luck with FedEx for things like tires. They are literally half of what UPS charges. Also, stay very far away from the UPS stores (nee Mailboxes, etc.). Many of those are Mom & Pop outfits and they set their own prices, which in many cases are so outrageous they must arrive at them through rectal extraction. I got charged $64 at a UPS store. Regular UPS for the same items (a pair of tires): $51. FedEx: $28. Greyhound (or any other bus line) is good for strangely sized things (exhausts?)