You might also consider another option: Park your car in Mammoth, and then hitch hike (or take the ESTA bus) back to Mammoth at the end of your trip. Everyone reports hitching from the Portal to Lone Pine is quite easy. Not sure about Lone Pine to Mammoth, but there is an awful lot of traffic on the 395, so it ought to work ok. Also, after your completion of the trail, it might be a day that works with the ESTA bus.
