Because they already have a deal with the post office where you can send something to your nearest Netflix distribution center without having to specify where it is. At the moment the only thing you can send there is Netflix DVDs, but fixing that is just a negotiation away.
It seems Netflix solves that by not requiring the USPS to actually deliver those DVDs, they just get them to a hub and Netflix workers fetch them from there (and occasionally simply check them and re-label them for deliver to another customer), but that seems a bit too complex and expensive for regular mail and packages.
