I think even a startup could do this. Could work with existing stations to promote themselves as being completely eco-friendly. They would add a small cost to each gallon of fuel (as andrewwhartion mentioned it should only be ~3-4%) and that cost would go to offsetting the emissions. The startup could charge another 1% or a lump fee and put the work into making banners / promotional material to advertise their eco-friendliness, and also handle the PR. In an eco-conscious city just the local PR could bump business enough to make it worth it.