Saving a tree a byte at a time.
This past weekend I went up for some wine tasting, and my friend exclaimed why is it that print at home confirmations always take two pages, and the second page is almost always irrelevant. Why indeed. Web designers should think about confirmation pages after purchases, e-tickets for planes, will call receipts, restaurant reservations, etc. to keep the content to be printable on one page.
As we increasingly have print your own documents, making sure they only use the paper necessary can make a big difference. Especially when you think that for that application you can save 50% of the paper used. Not many changes in processes can make such a difference.
So until web producers get it, look at your receipts in preview mode and only print the pages necessary. That would reduce the number of trees needed.