Printed matter, flyers, circulars, advertising.
Newsletters, bulletins, and catalogs.
Small parcels."
(via http://pe.usps.com/businessmail101/classes/standard.htm)
USPS Q2 FY2014 Revenue from Standard Mail: 4,228,705
USPS Q2 FY2014 Total Revenue: 16,732,651
(via http://about.usps.com/who-we-are/financials/revenue-pieces-w...)
Junk mail accounts for a quarter of USPS revenue. I don't suppose the USPS wants to encourage harassment of junk mailers.