Shipping guitars overseas is a lot easier than it appears. First of all make sure that the country you are shipping to allows for that type item to be imported, second order of business make sure the package size is within limits. Packages to most overseas countries that are of any big sizes or weight can be very expensive, for example; Fed Ex and UPS for the same package are about the same price and USPS will charge about HALF overall. Now with this in mind MAX insurance coverage through the USPS is around $400.00, which means if you sell a guitar for $1200.00 online through eBay it will only be insured for $400.00 thats it, they DO NOT insure for any higher amounts.

 I have a standard box size that works well, 20 x 8 x 50 inches, 106 inches total, this fits most countries legal size limits and fits for most types of guitars, electric and acoustic with a hard shell case. Basses and larger guitars will need to be dissassembeled and packed well to meet country requirments. I have included a list of counties and size chart curtosy of the USPS which is very hard to find on their site.This is their standard limit size, don't go over they will not ship! Note: if your coutry is not on the list it is classified as part of the all counties.

From the USPS site:

The maximum size limit for rectangular-shaped parcels of 42 inches in maximum length and 79 inches in maximum length and girth combined applies to all countries except as follows:

Maximum length and girth combined: 108 inches.

Canada
 
Hong Kong
 
Maximum length: 60 inches

Maximum length and girth combined: 108 inches.

Andorra
 Luxembourg
 
Austria
 Macao
 
Azerbaijan
 Malta
 
Belgium
 Netherlands
 
Denmark
 New Zealand
 
Finland
 Norway
 
France
 Poland
 
Germany
 Portugal
 
Gibraltar
 San Marino
 
Great Britain and Northern Ireland
 Slovak Republic (Slovakia)
 
Greece
 Spain
 
Ireland
 Sweden
 
Italy
 Switzerland
 
Japan
 Vatican City
 
Liechtenstein