Find the zip+4 for an address or what zip codes exist for a City.
Find Official USPS State Abbreviations
Find Official USPS Suffix Abbreviations
Find Official USPS Unit Designation Abbreviations
Find First Class Postage Rates up to 13 ounces for post cards, letters, and large envelopes
Domestic and International Package Rate Calculator (More than 13 ounces)
USPS main page
National Customer Service Center (NCSC)
You can contact NCSC at (800) 238-3150 for answers to your USPS questions.