A possibility to add user-defined surcharges has been added to the Simplair reservations module, as of verion 1.1.4.
These custom surcharges may include e.g. in-flight meals and drinks, Rent-a-car options, different tourist packages, etc.
Surcharges can be limited to specific connections or specific departure airports and/or destination airports. Also, surcharges can be limited to one way and/or round trips, durations of the round-trip as well as only a certain date period of booking and/or flights.