Just returned from Thailand and did something very similar: Landed at HKT and needed a car to Ao Nang, which is in Krabi province. We used Phuket Airport Limousine and Business Co-Operative (see their site) and were extremely pleased. They quoted us a r/t price that was considerably less than the hotel wanted. It was more than we could might have paid if we landed and bargained with the drivers at HKT, but we were landing late at night and didn't want the hassle. In marked contrast to the other drivers we had in and around Ao Nang, our driver was absolutely excellent -- 100% on time and knowledgable about the best routes, etc. Also -- and this is no small thing -- he wasn't a daredevil...we felt totally safe with him behind the wheel. Slightly under 2 hours from HKT to Ao Nang. We paid 5,400 BHT r/t, and it was worth the prumium to not have to worry about anything. If you book through them, ask for Peter.