A. The easiest way maybe to contact the merchant directly and ask the merchant to correct the error. If that route doesn't work, you can contact the credit card issuer directly - per the Fair Credit Billing Act, you have 60 days from the time you received the credit card bill that contains the error to file a dispute, and the creditor has to settle your case within 2 billing cycles.
During the dispute period, you do not have to pay the amount in dispute.
If the result is to your favor, that's great. If not, you can still file a complaint with Consumer Financial Protection Bureau, it will forard your complaint to the issuer, who must respond within 15 days.