Postal Explorer - This page on the USPS web site has links to a number of useful USPS documents in Adobe PDF format, including the Domestic Mail Manual (DMM), Quick Service Guides, and Postal Adressing Standards. You can search these documents by entering your search term in the box and selecting the document you wish to search in the check box. Clicking the link will open the document using Adobe Acrobat reader.
Undeliverable Mail: For information concerning what optional phrases that can be printed on the outside of a mail piece to specify how undeliverable mail is handled, click the DMM Subject Index, go to Standard Mail on p. 12, and then select undeliverable mail.
Tabbing and Wafer Seals: For information about tabbing and wafer seals, click Quick Service Guides and then topic 811 in the Automation-Compatible Mail section.
Domestic Postal Rates and Fees (simplified) - Contains rates and fees for most USPS services and mail classes except standard and non-profit.
Domestic Rate Calculator - Calculates postage for letters, large envelopes, packages and postcards based on source and destination ZIP Codes and weight.
ZIP Code Lookup - Provides ZIP+4 Code for an address and more.