I would be very surprised if an attorney filed suit over $500. If suit is filed, I would raise the defenses mentioned. You could always offer to settle the amount for less - say $100 or $200 or so - but make sure you protect yourself with regards to your credit report - meaning if you settle make sure you obtain documentation verifying such.
The credit card comes with a contract that governs how to deal with disputed charges. Some cards have a mediation process, some have protection plans, etc. Look closely at the agreement. That will govern whether you have to pay. In addition, if it doesn't look favorable for you under the contract, you can always offer something less that full payment to settle.
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