There is a free shuttle that leaves every 15 mins from the FDX ramp to the OAK Bart station and vice versa. The last time I used checked it was a red bus operated by one of the local park-and-ride companies. Take the Green or Blue line from the OAK station and transfer anywhere after the West Oakland station to the Yellow line. That will take you to the SFO terminal station. It costs approximately $3.