Exercise your billing error correction rights under the Fair Credit Billing Act as outlined on every statement you get.

Send a letter (only a letter will do--no 'phone calls, faxes, or emails) to the special billing error correction address on your statement (hidden away somewhere) and dispute the charge. More info at http://www.bankrate.com/brm/news/cc/20031111a1.asp

Note that a charge in the wrong amount is a billing error under the Act. And, once you assert a billing error in writing the burden of establishing the correctness of the charge shifts to the merchant.

If some FTers start doing this with DCC I suspect that will give, e.g., Hertz a costly kick in the ... and an incentive to stop. Billing error disputes are costly for the merchant. Plus, you are very likely to get your money back.