Yes, it can be done, bus from the airport to the port and on to a ferry, yup. Can be quicker to catch the bus than wait in a queue for a taxi (and cheaper!) they are every 20 mins at that time and I've seen the taxi queue snaking 150m down the road. Do the car hire thing on Formentera, but to be honest the bus service in September is reliable and frequnt enough to get you almost anywhere, (avoid the expensive "tourist buses"!) anthough I'd suggest a scooter to get to the far-out places.FFS wish I'd thought of the hire car thing before didn't even occur to me that you can't take a hire care from ibiza to formentera...! Silly me!
If we are scheduled to land around 3pm, realisitic to think we'll make Formentera for about 6?
Will be carless now so taxi to port and then will need to pick car up the other side.
There are specific transfer deals from the airport to Formentera, look at the Trasmapi, Balearia or Pitiusa websites (the ferries), could be good for a few people + luggage.
Example; http://www.trasmapi.com/servicios but I'd just use the 10 bus which I think goes directly to the port (used to....but the building work may have altered that, but it will stop pretty close even so).
My personal world record from stepping off the plane to arriving at la Savina is just over 90 minutes!