Abstract: A computer-implemented method of vending air travel services comprises determining a plurality of published fares each associated with a respective one of a plurality of root booking classes, said published fares being applicable to the purchase of air travel at a root service level. The method further comprises determining a plurality of derived fares each associated with a respective equivalent booking class that is one of the root booking classes, said derived fares being applicable to the purchase of air travel at a non-root service level, the derived fare for which the equivalent booking class is a particular one of the root booking classes being computed from the published fare associated with the particular one of the root booking classes by applying a price differential that is common across multiple ones of the root booking classes.