The difference for me is that that getting gas is a REAL time-taker-upper for me, and that I really have outsourced this job in the past. I've never outsourced getting quarters for laundry, because if I'm going to do that, why wouldn't I just outsource laundry altogether?
FWIW I could see myself using this service. What would probably change it from a "maybe" to a "definitely" for me is if you offered not just fill-ups but oil changes, detailing, mechanic visits - i.e. all vehicle maintenance.