Well.  You're gonna need a larger car for your skis and all your gear.  How long do you plan on staying there?  I have never seen a rental car place in Mammoth.  I would bet there is not one because originally the airport was private.  Now, I would imagine there being a rental car place.  Let me go check right now.  I have a feeling you are out of luck on returning the car in either Mammoth OR Bishop.  I checked Hertz and Avis, two largest in the states and nothing came up.  I would say, since you're going to NEED something like a ford explorer if you're going more than 5 days it's going to cost more.  I know gas is cheap now, but you can expect at least 60 U.S. smackers for gas there and back.  Probably two tanks (one there and one back)  I would say fly and ride shuttles around town, it is WAY more fun!  I just checked Greyhound and there are NO buses to any area close to Mammoth (Bishop, Mammoth Lakes, June Lake).  So you're in for a rental car or plane and ride around on the bus.  If you're there for over 5 days you're a LOT better off flying.  If you do decide to go with the rental car that Quality Inn has super sweet parking!  It's a garage under the hotel and we were 15 seconds from our car.  It was Sccchhhwwwweeeettttt!  Not a nice hotel, it's good if you want to sleep and eat a free breakfast.  They're good with foreigners too.  When me and my Dad were there almost all the hotel had French people.