This may not sound as familiar to you as it did to me, unless you are also a student of medieval history; but the human race has been here before. In the 15th and 16th centuries (that's the 1400s and 1500s), those of extreme sinfulness could get indulgences from the church, which would cleanse them of sin for specified periods of time. They had to do this so they could attend church and receive the Eucharist.
Indulgences occur when the Church, acting by virtue of its authority, applies existing merit from the Church’s treasury to an individual. The individual gains the indulgence by participating in certain activities, most often the recitation of prayers.I'll confess to being charmed by the notion of an existing treasury of merit; especially considering the general moral condition of the Catholic Church in the period in question. The church said that no money could change hands in exchange for indulgences, of course; but, also of course, it did; and the scandals associated with the sale of indulgences were part of what brought on the Reformation and the rise of Protestantism. Martin Luther's 95 Theses were all about the sale of indulgences.
The difference is interesting here: the medieval indulgence buyers were trying to save their souls, or at least reduce their stay in Purgatory. The carbon offset buyers are trying to do - what? Save the world? I'm afraid all they're really trying to do is appear "greener than thou" to their neighbors. But we've changed the definition of "sin" rather substantially, haven't we?