I recently had a guitar shipped over, the postal rate was 70 dollars for a declared value of 1000 dollars, I had to pick the guitar up and had to pay the vat on the 1000 dollars. Shipping took 3 days, could have done it cheaper with up to 10 days shipping.
Make sure the case is left unlocked but securely sealed with loads of thick tape, the reason for the case being unlocked is if customs want to check the contents they will force it open.

Last bit of advise do not try to be smart and under declare the value by too much, customs have a manual of all guitars, even vintage ones in order to check up on what you say it is worth.
If they catch you under declaring the value they have the power to confescate the guitar which then becomes the property of the crown, you then have to try and buy it back.You also get a fine.