There are volume discounts and they are negotiable to some extent. The thresholds are fairly high e.g. USPS cites 5000 units per year. However, if you are talking about complex shipping requirements, then it is difficult to qualify for lower rates.
The large mail order houses presort their packages and deliver them to regional hubs run by the major shipping firms. In return they get discounts. Some businesses like book dealers literally make their money by charging full shipping rates and then get a discount by shipping at least 300 books a day. That is why they can make money selling books for one penny with $3.99 shipping fee. They are only paying a fraction of the $3.99 that you or I would pay if we went to the post office with one book. (This info is circa 2010).
I do know that whatever that number actually is, no banjo bridge maker will ever come anywheres near enough to that magical number... Like Scott said, it takes VOLUME and volume means huge numbers - huge enough that only a few people, regardless of the kinds of product they sell, will ever find out...
Heck, I don't even get Air Miles on the postage I charge folks