No joke, I just said the other day, "Too bad they don't have EcoModder T-Shirts".
Either a Tee/Jersey or a Polo sounds good -- maybe have both available? I'd buy one
Kevlar has an excellent point: the window clings are hard to see -- a bright white bumper sticker would work well to "Rollingly Advertise". It's hard to see the one on my back hatch glass (bad angle).
If people are concerned with sticking something to the finish, I've used a roll of magnetic material to stick the sticker and trim to fit. You can remove it to wash the vehicle, or move it around to prevent paint fade issues. Also works well for political stickers (especially if the candidate lost).
RH77