It's a violation of what's called the "pre-existing duty rule" in contract law. If you're in the US, you paid the 5%, and got your money, you can sue in Delaware Small Claims Court and try to get the 5% back.

Mt. Gox has a legal presence in Delaware, which they established to register with FinCen. So they can be sued there. Suing a non-US entity via their US presence in Delaware is a routine legal event.

Advice from a lawyer would be helpful, but you're not required to have one for a small claim.