Can't help at all with shuttles I'm afraid. I wonder if there is some kind of directory or link from the Sanford airport website that would direct you to some reliable companies? Also try posting this on the US portion of the disboards, you may get more knowledgable answers there!
Have you worked out how many miles it would be from Sanford to your resort? May be worth looking into cab prices as a last resort option... May not be as bad as you think.
Hope you get some answers soon... Let us know how you get on. I would imagine if I travelled solo too I'd want everything sorted and booked before leaving the uk.
We have used America Coach several times as we usually fly into Sanford and Never hire a car.
I can't remember the cost as we havent done it for 2 years but they will take your booking by telephone.
When you arrive you just go to the desk and they get you on the next available shuttle. sometimes they are coaches and sometimes smaller vehicles - i guess depending on how many people are going to a similar area at the time.