Phil my friend I buy my guitar shipping boxes from U-Line: http://www.uline.com/ProductDetail.asp?model=S-4922&searchedkeywords=guitar and they are $7 each in small qualities with a minimum of 5. The boxes are nothing special but work fine for me.
I have used USPS Priority Mail and was pleased with them. I have also used FedEx ground which in my area can reach my finisher in only a day. Both charged me in the neighborhood of $35 one way.
The finisher will pay the freight back to you and just add it to your bill.
I pack the guitar with neck on, no strings of course, in a guitar case and then I place some 1/2 hard. pink....., foam around the guitar case to make it fit snug. For the rest of the voids I throw in my dirty laundry. Some finishers will not do your laundry and return it folded and sealed in plastic......
Not that this will help you but I am fortunate that I can drive to my finisher in about 1.5 hours and it's always a great time to see Joe White and what he is up to these days. So actually I am no longer shipping my guitars favoring to visit Joe and Hang out a bit.
