If there's any likelihood a problem may arise with a purchase, use a credit card, if possible. You'll have more recourse than if you pay with a check, cash or most other forms of payment.
The federal regulation that conveys these rights is the Fair Credit Billing Act. It applies to "open end" credit accounts, such as credit cards and other revolving credit accounts. It does not apply to auto loans or other accounts that you repay on a fixed, installment basis.
The FCBA's rules apply to "billing errors," which include:
— Unauthorized charges;
— Charges that list the wrong date or amount;
— Charges for goods and services you didn't accept or weren't delivered as agreed;
— Math errors;
— Failure to post payments and other credits, such as returns.
If you detect a billing error on your statement, write to the address it provides specifically for that purpose, which in most cases will not be where you send your payments. The address and information the creditor needs will be shown somewhere on the statement.
The letter must reach the creditor within 60 days after the first bill with the error was mailed to you. Consider sending the letter return receipt requested and keep a copy of all documents.