That still, as far as I know, is outside AirBnB's Terms of Service and entirely outside their payment system. I suppose you can communicate prices and payment options through AirBnB's message system, but that will put you at risk with AirbnB if things go awry and guests complain.
Personally, I think this sort of optional service would be an excellent addition to AirBnB. Perhaps you might suggest that to them and see if they eventually fold it into their booking and payment system. On the other hand, you should also be aware that charging for transportation w/o proper license/permits probably falls under your municipality's regulations, so you might be at jeopardy there.