What a wonderful idea!
I think many people would be more encouraged to send paper letters if they knew that a few cents would go to carbon sequestration, especially if they were somehow made aware of the fact that mail carriers already travel to every house, so there would be no added cost (and isn't in general) to delivering an actual letter now and then (to go with all those advert mailings we all wish we could get rid of).
I think many would actually prefer to give more more per letter. Even 4 or 5 cents seems very reasonable to me.
A name suggestion: "stamp out carbon"
I don't have time to do the research for this now (building a geodome shelter to take to SR in a week), but I hope you can get ready in time for your meeting--As Bernie would say, this is huuuuge.
all the best
(As to the Standing Rock connection, even though I am an absolute supporter of the Water Protectors, I guarantee that there is no chance of connecting that movement to anything that is in any way related to the government--and may not have the desired results even if it was a possibility).