Well, first off, they have a special arrangement with the USPS, so their shipping costs are low due to volume. The deal is kept very private. That brings me to the other part of Netflix, it is a very, very secret organization. They constantly fear corporate espionage, because they apparently have a very cost effective and special way to sort and send DVDs inside their facilities. I read a news article where the journalists weren't even allowed really to talk about what goes on inside a Netflix sorting facility.
And as mentioned above, the system thrives on people checking out movies and not returning them for weeks. High volume customers are rare and that's why most people get "throttled" eventually.