I find that a little hard to believe given that you can/could service Clipper cards at any Walgreens and a bunch of other retailers (e.g. pharmacies and hardware stores).
I was just North of Golden Gate Park. The nearest bus stop was 1 minute away. I wanted to take a 20 minute bus ride. The nearest Walgreens was 30 mins walk away (Geary and 42nd/43rd). The nearest street with shops was 12 mins walk away. I don't know whether any of those shops offered Clipper card top-up.
