fees

Simple question but doesn't seem to have an easy answer. Looking around here and other pages I can't wrap my head around it. Isn't there a percentage or specific ratio that's official? Looks like it depends greatly on how the guitar or other gear are sent and from where the package is coming...

Anyone ever ship to England from the USA through USPS? They said it would be about 80 dollars to ship a guitar case. Are there any other hidden fees or customs charges or anything. I've never shipped internationally before. Thanks!