Google Maps show that one can go by bus 84 to Cabrillo National Monument past the fee station. How does one pay the entrance fee? Does the bus stop at the fee station and then proceed further?
It looks like people who take Bus 84 into Cabrillo National Monument get in free.
A Trip Advisor poster from 2017 notes that he got in free when riding bus 84 in.
Surprisingly, this bus route doesn't seem to be very well utilized, or this perk well publicized. That same year, SDMTS apparently tried to discontinue the route, and received a lot of public outcry about it. The Point Loma Association also notes that bus 84 riders get in for free, and that year advocated for the service to be expanded to weekends and holidays rather than curtailed.