The trick with reducing shipping costs is making sure you use the smallest box possible. They figure the cost with not only the wieght but the overall dimensions of the box. I typically use the uline.com guitar boxs which are basically the size of a strat case. 18x6x45. Tall guitars like Explorers and Basses you need the 50" tall box and that makes the price go way up. Shipping to your closest side of the country too is obviously cheaper. I can ship stuff for reasonable prices to anywhere on the east coast, but California, Arizona, etc get expensive.
What I do, is I have a free account on FedEx.com. I print up my shipping labels there, and then drop the box off at Kinkos/FedEx. I get a slight discount for using an account. Also, insurance and getting signatures increases the price as well. I've shipped about 50 guitars in the last couple of years and by far FedEx is the cheapest. Shipping usually costs me anywhere between $25 and $40, not including the cost of the box, if I buy them form uline, which adds about 7 bucks to my cost.
Also, FedEx owns Kinkos now, so it's not like its a third party store. As long as all you do is drop off a pre-labelled box, there should not be any difference in price if you dropped it off at a FedEx distribution center.