An important element to remember concerning two-part tariffs is that the product or service offered by the firm must be identical to all consumers, hence, price charged may vary, but "not due to different costs borne by the firm", as this would imply a differentiated product. Thus, while credit cards which charge an annual fee plus a per-transaction fee is a good example of a two-part tariff, a fixed fee charged by a car rental company in addition to a per-kilometre fuel fee is not so good, because the fixed fee may reflect fixed costs such as registration and insurance which the firm must recoup in this manner. This can make the identification of two-part tariffs difficult.