What makes a letter? (html)
Mailing Standards of the United States Postal Service Domestic Mail Manual (DMM)

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USPS Service Standards Map

Nondenominated Postage (html)

Customer's Guide to Mailing (PDF 747kb)

Suspicious Mail or Packages - USPS Poster 84 (PSN 7690-07-000-7097) (PDF 187kb)

Immediate Response Actions Suspicious Mail and Unknown Powders or Substances - USPS Poster 205-A (PSN 7690-08-000-1683) (PDF 195kb)

USPS 5 year plan (PDF 3M)

Stamps to Beware of (PDF 279kb)

Current International Stamps (PDF 204kb)