Zoe, We reserved a car thru Alamo, they are running a special Hawaii rate. $28 a day for full size, $37 for mini van. Rate code is J9. They told me the airport is closet to Lahaiana and that they do have a shuttle. Let me know if you find out different.
We have reserved cars for all of the stops on this cruise except for Kona (no rental agency except at the airport and that's too far away). We used Dollar and they insist they have pier pick up and shuttles on each location! If you need it here is the toll free number for the central Oahu location and they can help you with each of the Islands. 1-800-367-7006. We went for broke and reserved convertibles everywhere but Oahu, gotta have somewhere to put all that luggage! Good luck and I hope you find what you are after.