Check around for other car-shares in the area. We've got two in Philly, Zipcar and something called Philly Carshare. The second is actually a free membership, you only pay when you use the car. Perhaps San Fransisco has something like that.
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"But I want you to remember, I intend this breast satirically." Susan from Coupling
Also if you google zipcar discount/coupon/code you should be able to find a coupon either waiving/lessening the annual fee or for some free driving time.
Good luck!
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“Ginger Rogers did everything Fred Astaire did. She just did it backwards and in high heels.” Ann W. Richards