I will say, however, that I appreciate Slate actually approaching this issue--many people seem to think that "electronic" equals "green" because there's no paper, no delivery, etc, etc. They forget, however, that there is a vast, (currently) energy dependent system that supports the internet.
I really try to cut down on the easy things. Getting rid of single serving containers from the supermarket, reusing containers, etc. A nalgene is a bottle just like the bottle of bottled water. While an environmental scientist might be able to calculate out precisely what Netflix and Blockbuster use, there are some no-brainers that most people don't do - like turning the thermostat down and putting on some nice wool socks. Plus, I'm cheap and those energy/waste savers also tend to save money!
1. USPS truck vs. your car. The USPS is going to be coming by your house whether you get Netflix or not and the weight addition is negligible. You driving to the video store causes an increase in emissions.
2. Video store vs. factory. The video store has a lot of wasted space in it. All those aisles to browse in, all that lighting, AC, heat, etc. running from 9am-midnight or later. Netflix is probably open from 8am-4pm if not less in a factory that can be made really space efficient.
3. Labor. Here's an interesting premise: we all pollute by breathing. If we are doing something more productive, we can better justify that pollution. For example, no one would say that EMTs should walk a person to the hospital in an emergency - the use of the ambulance is justified by the importance of the situation. However, walk for less important things (if you live in a suitable area) to lessen pollution. Likewise, if we can reallocate labor to more important/productive things, it's better than if we have labor sitting behind a counter with no customers for hours on end. Netflix uses labor efficiently. Package everything up, get them out, go home. Vs Sit behind a counter for lots of idle periods doing nothing.
Netflix just works off a much more efficient model. More importantly, I think we're seeing that in a decade Netflix will be a purely virtual model. They're a smart company getting into the video on demand business so much. That will really cut down and, really, in this day we don't need physical media to transmit audio/video.