[TimJRobinson]

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.

[andrewwhartion]

I think there's something in this...

The person behind the cash register could even ask, "Would you like to make your 'gas/petrol/fuel' today carbon-neutral for an extra $2.45?".

I can see that working with very little capital. It gets sold the same way as the Snickers bars next to the checkout.

[cpprototypes]

A lot of gas stations here ask "Do you want a car wash for $x.xx" directly on the gas pump after you slide your card. If gas stations just added "Do you want carbon neutral for $x.xx" I think many would push yes in the right markets and a reasonable $x.xx value.