While that may be true in this context, for those who use Zumbox, it does represent a green solution. Because it does help people or companies to be green by reducing their use of paper and all of the related impacts that come with that.

What we mean in the above is that systems often overlap with other systems. The USPS is a system that uses several others: energy, transportation, communications, etc. These stand on their own and aren’t reliant on the USPS. If you strip those out and look at the core system, you find one that delivers paper documents as well as parcels. That particular system, which is certainly large enough to be a macro concept, is unsustainable because of the gross misuse of natural resources (trees) and the ecological impact (waste). You can address the system by making it more efficient (i.e. green) by reducing the number of trees it consumes, but greater efficiency alone will never make it sustainable. An entirely new system needs to be developed, where sustainability is part of its DNA.

Cradle to Cradle defines sustainability in absolute terms quite clearly and convincingly. When water can exit a factory cleaner than when it entered, that is a sustainable system. The factory may run on coal power, which is not sustainable, but the system that processes the water still would be.

On the other hand, it’s conceivable that if all of the paper the USPS delivered could return to the soil and decompose organically with no toxins or ecological impact, then the existing system could become sustainable.

You say that sustainability is a macro concept, and it applies broadly to entire systems and infrastructures.

But in the Zumbox example, you say “However, the actual system that Zumbox addresses is not energy but rather paper mail.” Here you draw the line on what the “entire system” comprises.

I am not necessarily disagreeing with your point, but I think it is impossible to create absolute lines when it comes to sustainability, especially using the lines you drew here.