Every flight is charged Navigation Charges for passing through various FIRs, UIRs and or National airspace. The charge is based on declared MTOW for the aircraft in question which is why many airlines declare and operate their aircraft at MTOWs below the manufacturers book figure.
Going to the Canaries, it is possible to avoid France, Spain and largely Portugal by flying on a "Tango" route, airways T16 etc, which are Oceanic airways, BAGSO being one of the Oceanic Entry points for Shanwick OCA. Thereafter you get doen to Santa Maria OCA and somewhere along the line rejoin the domestic airways set up. The obvious departure for that routing, well West, is via WAL VOR. It is a longer flight time and therefore burns more fuel but saves on Nav Charges. One of the many variables in maximising profit on a given flight.
They are also excellent on the 360 days a year there is some kind of French ATC "issue"!
