A method and system for making roundtrip air flight reservations includes an end-user receiving a response having departure dates with a lowest cost outbound fare for each date and return dates with a lowest cost inbound fare for each date, outbound flight and inbound flight segments each priced with One Way Combinable fares, the One Way Combinable fares can be priced independently of all other One Way Combinable fares (i.e. restriction on covered segment, fare amount can be doubled to price the Round Trip) and can be combined together with any other One Way Combinable fare (i.e. no combinability restriction) in order to form a guaranteed pricing solution regarding IATA regulation both in domestic and international contexts for complex segments (i.e. based on non-direct flights).