The bay bridge penalty charge to people without a fastback tag.

I am a TX driver and i went to CA for visit, in Jan I drove thru the Bay bright locate in San Francisco Bay area, i was being ask for a fee to cross the bridge, which is $5, and person who work for the company in the boost said if you don't have a exact change in hand you can pay 5 dollar online later, when i receive the mail bill, there is 25 penalty said without a tag violation? and by the way they never mention anything even a sign in the bridge when i passed it.