I shipped a vehicle, although it wasn't to bypass exhorbitant rental rates.
The other thing you need to factor in is shipping time; it can take many days ir not longer to ship it back/forth, leaving you carless.
In addition to not having anything in the car, you also need to ship it with almost an empty tank of gas.
I wish the interisland ferry was back! Even interisland shipping of vehicles can be timely and expensive.