You may use the program free of charge indefinitely only if
* You take at most 4 flights (2 return flights) in any 12 month period
* AND you do not own or regularly drive an SUV (sports utility vehicle).
There was some tradition with shareware authors years ago of asking for things like postcards, and a lot of web software asks for links back, especially since the rise of PageRank. And I definitely recall little applications created by independent developers that were charity-ware, asking for donations to a cause. But this is the first time I've seen a software license specifically addressing climate change.