deliverables: (1) mileage credits earned and (2) air transportation. We value each deliverable on a standalone basis. Our estimate of the selling price of a mileage credit is based on an analysis of our sales of mileage credits to other airlines and customers, which is re-evaluated at least annually. We use established ticket prices to determine the estimated selling price of air transportation. We allocate the total amount collected from passenger ticket sales between the deliverables based on their relative selling prices.

We defer revenue for the mileage credits related to passenger ticket sales when the credits are earned and recognize it as passenger revenue when miles are redeemed and services are provided. We record the air transportation portion of the passenger ticket sales in air traffic liability and recognize these amounts in passenger revenue when we provide transportation or when the ticket expires unused. A hypothetical 10% increase in our estimate of the standalone selling price of a mileage credit would decrease passenger revenue by approximately $48 million, as a result of an increase in the amount of revenue deferred from the mileage component of passenger ticket sales.

Sale of Mileage Credits. Customers may earn mileage credits through participating companies such as credit card companies, hotels and car rental agencies with which we have marketing agreements to sell mileage credits. Our contracts to sell mileage credits under these marketing agreements have multiple deliverables, as defined below.

Our most significant contract to sell mileage credits relates to our co-brand credit card relationship with American Express. In December 2014, we amended our marketing agreements with American Express which increased the value we receive under the agreements and extended the term to 2022. The amended agreements became effective January 1, 2015. The deliverables under the amended agreements are substantially similar to the previous agreement. We account for the amended agreements consistent with the accounting method adopted in September 2013 that allocates the consideration received to the individual products and services delivered based on their relative selling prices (discussed below). The increased value received under the amended agreements increases the amount of deferred revenue for the travel component and increases the value of the other deliverables, which are recognized in other revenue as they are provided.

In September 2013, we modified our marketing agreements with American Express, which required us to change the accounting method for recording SkyMiles sold. Under the previous method, the embedded premium or discount was allocated to the residual products or services in a combined transaction. The new method allocates consideration received based on the relative selling price of each product or service. We determined our best estimate of the selling prices by considering discounted cash flow analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) the rate at which we sell mileage credits to other airlines, (3) published rates on