You must have missed my post in the Hawaii forum:
Where will you be staying? Is there any way you could take a shuttle/cab to your hotel and rent in the morning?
That's what we do. For example, the Westin will send a car gratis (or have one waiting if you notify them in advance). I know many of the other major resorts (and some of the smaller ones too) in the immediate vicinity, offer free pick-up and delivery to Kapalua West Maui Airport (JHM) as well.
We often save on car rental doing this. We plan several days as "resort hotel area only" days, and then have them shuttle us back to JHM for a car rental when we want to do our longer haul off-property days. It's one of the main reasons I love JHM. Saves on car rental and you're at your hotel within minutes of landing.
On the trips when we stay at our condo at Napili Bay where they don't offer pick-up, we just cab it. It's so close it's not much in cabfare, if I recall.
And it's just cool to arrive Maui at JHM! That prop job comes in and dives down on that little airstrip...fun! Great way to arrive on an island! Beats the far-away industrial experience of Kahului (OGG) and sometimes having to fight the traffic from OGG to the Kaanapali area.
Although I would suggest you call the Westin to re-verify they still offer this service gratis.