The last time I was there, I noted that the handling agents office was located airside just to the left of the exit gate. You can go in there for whatever you want and still be airside.
You are changing your tack DFC, correctly at last. And Biarritz is not the only place, by far, where this is so, and that includes the airport I have in mind.
In fact one can pay for most things remaining airside, nowadays, using cash or plastic.