i would stick to the major rental companies. you can drop the car off at the san francisco airport,

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i would stick to the major rental companies. you can drop the car off at the san francisco airport, because all the rental agencies are there. (the aiport is located in south san francisco, so make sure you have a ride back into the city.) i m not sure which ones have a drop off charge, but i can tell you that car rental companies that know they won t be getting the car back (one way rental), try to use the worse car so if you don t like the car your offered ask for another. all rentals have ac, and it won t be to hot in san francisco, so no worries there.
It won t be super hot that time of year but cars do/will come with A/C. If you are looking for a simple red lion inn car the cheapest way to go about it is Avis. Enterprise and Budget are about the same price but I personally have had the best experience with Avis. I typically request an economy car and they always seem to find a way to upgrade me for free every time.