Over the years I have disputed numerous forex charges and overseas charges with excellent results. IMHO, it is a good idea to dispute the entire amount and let the merchant try to justify it.

Be very sure you mail your written dispute to the special billing error address on your credit card statement (NOT the address to which you send payment) and that you have proof that the letter was received (enclosing a check with the letter that notes on it "enclosed with billing error letter of xx/xx/xxxx" is a good idea--if you can get a copy of the cashed check) and that you do this within sixty days of the date the statement with the disputed charge was mailed to you. DO NOT rely on phone calls, faxes, email, etc to dispute the charges as only a mailed billing error dispute counts under the Fair Credit Billing Act.