The substitution of information for stuff is one potentially winning strategy for creating more value, while at the same time not putting impermissable burdens on underlying natural systems. The second is biotechnology, for the same reason, and the third is nanotechnology, the emerging science of radical miniaturisation. A letter requires you not only to cut down trees, and type a physical object called a letter, which you then in some way transport to someone who drives a car, and puts it in a plane, and puts in a railroad car, and delivers the letter to the intended recipient. A lot of stuff has to happen in order for a letter to work. E-mail just moves electrons.
Electrons are cheap, renewable, and sustainable.