That's because the tolls are entirely electronic over the golden gate bridge, though they're only assessed in one direction. Incidentally, If you get a rental car in SFO it comes with such a "gizmo" needed to pay the tolls, and when you return the car all assessed tolls are added to your rental bill.
If you don't have a gizmo, they scan your license plate and send you a $25 penalty if you aren't registered in their system somehow (with a 48-hour grace period in case you accidentally get stuck crossing the bridge, it happens..)
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