Originally Posted by
flyermatthew
The BART ticket machines don't take credit cards for less than $20
Not true. Though at first it does say $20, you have the ability to decrease the amount using the touch screen. If all you need is a roundtrip between SFO and downtown SF, you can most certainly buy a $16.20 ticket just by decreasing the amount to that exact amount.
Also, I believe fares for BART can now be made on the SF wide Clipper Card. If you plan to go to SF often, it might be easier for you to get a Clipper Card and charge that up.