Our company travel policy specifically states "do not buy fuel from car rental agencies". If I'm rushing to catch a flight and miss the extra charge, it's an extra step for me to have to call customer service and get them to take off the charge as my company no longer reimburses me for it. I understand the need for it but irritating nevertheless.
Due to the automatic fuel charge Avis has now dropped to being an alternate if Hertz, National or Budget (in that order) don't have cars.