USPS:

Domestic Mail Manual

http://pe.usps.com/text/dmm300/dmm300_landing.htm

Rate Calculator

http://dbcalc.usps.gov/ieframe.htm

Zip Code Locator

http://zip4.usps.com/zip4/welcome.jsp

Quick Service Guides

http://pe.usps.com/text/QSG300/Q000.htm

Move Update Standard

http://pe.usps.com/text/qsg300/Q230a.htm

Ancillary Service Endorsements

http://pe.usps.com/text/qsg300/Q507d.htm