At present the function on the Air Mad website which should allow you to book a domestic flight at a discount in combination with an international flight is misbehaving in quite a weird way.
If you try to book a one-way domestic flight in combination with a Paris-Tana return flight, the site calculates a price of over 500 euros for the domestic segment (price if you book it separately, without the discount: 199 euros !)
If you try to book a return domestic flight in combination with a Paris-Tana return flight, the site either tells you that all flights for the domestic return leg are waiting list only and refuses to take your booking at all, or it calculates a price of over 600 euros for the domestic return flight (price if you book it separately, without the discount: 398 euros !). Or on occasion it does a combination of both.
Until Air Mad get around to fixing this, your best course of action is to book your international flight online and then either phone their Paris Call Centre or wait till you get to Madagascar to book the domestic flight(s).
Reason for the foul-up may be that they’ve added a new link “Multidestinations” to the main booking mask, to simplify the complicated procedure that I’ve described elsewhere on the forum.