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United States Postal Service
As an independent agency of the government, the United States Postal Service homepage offers information on zip codes, address change, stamps, online billing, and variety of other topics.
Smithsonian Institution
National Postal Museum: Decode the Barcode
Do you know what the barcode means at the bottom of an envelope? See if you can figure out how to crack the code that the postal service uses to deliver mail in this fun activity.