In California all employees must make at least minimum wage prior to tips, so do not feel that you need to leave a tip to make up their wage (most places will pay higher than minum wage prior to tips).
In my opinion CABs are about 10% tip, less if I feel the driver has tried to take advantage of me by not taking the fastest route.
For shuttles, it is usually considered customary to give $1 to $2 per bag that the driver has handled. Possibly slightly more for extremely heavy, bulky or difficult to handle luggage.