We guarantee that your credit card will be charged only once for the amount of an order. It is possible that the amount may be shown twice on your statement: the first entry corresponds to the payment authorization request that is made when you place the order; the second confirms the actual charge.
For AMEX credit card users, please note that the AMEX credit card refund policy is different from others; it has several steps where the initial sales transaction is refunded and then the remaining amount is charged as sales. (100 initial sales - 100 refund + 80 remaining amount charge). We do not have the authority to charge any credit card without the customer's consent.