The easiest and cheapest is to book the airport transfer in advance. The cost is 15 euros from 1 to 3 people to pay at the DEPARTURE of the riad. We must send the details of the flight: time and flight no.
The taxi will be waiting for you with a sign RIADS KARMELAS. It will leave you on the place Ben Saleh which is at 1 mn on foot of the riad. If you book a taxi with a local agency or at the airport, 
the taxi will leave you LOIN of the riad and you will lose yourself. The same goes for excurcions. The mini bus will NOT come to the Ben Saleh Square and you will have to walk far to reach your Excurcion. 
It is easy to book your Excurcions ON the spot at the riad upon arrival (no need to book in advance)