I think you are on the right track. I think there was a post here on HN the other day about needing a trillion trees to counter CO2 from human activity (or something like that).

So what if online retailers like Amazon had some sort of opt in where you could donate $1 extra with your order to get a tree planted (or it would go to that). It probably would not be enough to get you to a trillion trees but if a lot of online retailers offered that at least it would give people something they could do easily.