Well, if you are going to rent a car, wouldn't be cheaper to do a one way rental and drop it off in Wittier? There is an AVIS rental agency about a block walking distance from the ship. Rent it in Anchorage and drive yourself to the ship and cancel your transport bus.
I'd look into it. Getting to Whittier is easy. We did the reverse on our Northbound...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013