Postage costs were spread around among volunteers. Some people have more time than money, some have more money than time. Personally, I paid for about 250 postcard stamps, and got a roll of 100 from the group. Others got more or less rolls of stamps.
Cynicism among folks who care about climate change is understandable, and widespread, which is a big motivation for sending postcards targeting climate voters. The data shows that we tend to vote less than the average voter. If we really want the political changes that we say we do, then we need to show it and take action.
It’s entirely possible that it is too late to do anything about climate change. But if we act as if it’s true then we make it a certainty, where now it is only a probability.