Sounds like you will be picking up the boat on STT. We've chartered with CYOA several times and have found it very cost effective and convenient to pick up an Avis car at the airport. We then pick up provisions at Plaza Extra. You will pass Cost U Less on the way and could certainly stop there as well. Stick all the provisions on the boat and the next day return the rental car at the float plane dock and walk the 50 100 yards back to the charter base.

We have also provisioned at Pueblo which is a moderately long hot walk and a "taxi" coming back. The provisions you can get to with the rental car are in my opinion much better. And essentially for the cost of the taxi ride from the airport to the charter base plus the taxi cost from the Pueblo market back to base you will have paid for the rental car (I think in June the rental car including insurance was something like $85). So why not? Just make sure you think clearly at every intersection 'cause you're driving on the wrong (sorry my British friends...) side of the road. It's about the only place I ever pay for the optional insurance on the rental car just to make sure.