Access to the web service at
www.flightroutelookup.com/FlightRoute is, and always has been, open to all. No individual or organisation is prohibited from using it, and no negotiation is necessary. The only condition of use is that valid (not dummy) data should be entered in the search fields.
Having said that, I'm amazed that AirNav seem to want to base RadarBox route data provision, even partially, on a free-of-charge, pro bono service that was created purely for enthusiasts.
As AirNav know only too well, you get what you pay for :-)