Sign up once, then just add a single REST call to your billing flow.
Edit: apparently digitalhumani doesn't cream off.
Our solution is 100% free. We are a group of IT-oriented volunteers concerned about climate change, and we pay ourselves for the costs of the solution (which is quite cheap though).
Our RaaS (Reforestation as a Service) API was built because most reforestation organizations do not have APIs. These organizations know lot about trees, climate and reforestation, less so about IT. We help them get the money they need to plant the trees (1$ per tree) by providing the API for free.
The API is used in lots of ways right now by our clients: Planting a tree when someone subscribes to a newsletter, buys something, refer a friend for a mobile app, fills a survey, charge their electric car, etc.
It is much easier for our clients to just choose a reforestation project they want to donate to and add the call to the API in their code.
Why not just donate the equivalent of 1-year amount of your [whatever event-triggered actions] directly to one of these Reforestation projects? It saves your time, the cost of running server (albeit not much) while doing the same amount of help.