According to the http://www.usps.com/communications/newsroom/postalfacts.htm, it handles “212 billion pieces of mailhtml_symbol(odquote). Assuming that's a yearly (and uniformly distributed) volume, it corresponds to around 212/365 = 0.581 billion pieces a day. And that daily volume isn't observed in one place, but distributed unevenly around the country, but you should get the general idea: it's important to be able to route lots of mail quickly, correctly, and cheaply.
This page last modified on 24 January 2006.