Don't waste your money on any of the shuttles you have to pay for to get to Davis from the airport. Yolobus works fine and is super cheap.
It costs $1.50 on the way there and on the way back it will be free because you will have gotten your student ID, which gets you free bus service. It leaves once an hour.
Make sure that when you're leaving the airport you take 42B and when you're going to the airport you take 42A. They both go to the same place so if you get on the wrong bus you won't be screwed, your trip will just be unnecessarily wrong.
For more info go to
Yolobus - Routes & Schedules