When renting with enterprise, its only like a $40 drop fee; at least when I checked in March using a corporate code.
That being said, the raid is that the LAX and LA area enterprises are a major annoyance when returning, especially one way, about gas and scratches/dings. This comes straight from the SD enterprise associate.
When doing a one way rental with enterprise, you do need to call them directly. I would suggest making a reservation online, and then calling them to modify it to a one way rent; they may just cancel and make a new one, but you might as well, especially just to check the online price, which they usually match over the phone.