A photo is taken of your southbound trip's license plate and an invoice will be mailed. $8.40. You can prepay by an account.
The toll depends on a number of things but likely for you it's $8.70 and you pay online later - there is no toll booth. See https://www.goldengate.org/bridge/tolls-payment/
No toll now looks like they might have one in the future putting up toll booths
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