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Every address in the United States belongs to a specific zip-code district, and each zip code has five digits. Some addresses include a second sequence of four numbers following the first five numbers, but although this speeds mail delivery for large organizations, it is not necessary to use it. Each zip-code district has at least one post office, where you can buy stamps and aerograms, send parcels, or conduct other postal business. Occasionally you may find small stamp-dispensing machines in airports, train stations, bus terminals, large office buildings, hotel lobbies, drugstores, or grocery stores, but don't count on it. Most Americans go to the post office to buy their stamps, and the lines can be long. Official mailboxes are either the stout, royal blue steel bins on city sidewalks or mail chutes on the walls of post offices or in large office buildings. A schedule posted on mailboxes and mail slots should indicate when the mail is picked up.
First-class letters (under one ounce) sent within the United States cost 37 cents; postcards are 23 cents. A one-ounce letter to Canada or Mexico takes a 60-cent stamp and a postcard 50 cents. Letters sent airmail to all other overseas destinations cost 80 cents for one ounce or less (this rate category is now called airmail letter-post, and surface mail is now called economy letter-post); airmail postcards to other overseas destinations cost 70 cents. For 70 cents, you can also buy an aerogram—a pre-stamped sheet of lightweight blue paper that folds into its own envelope.
If you wish to receive mail while traveling in the United States, have it sent c/o General Delivery at the city's main post office (be sure to use the right zip code). It will be held there for up to 30 days. You must pick it up in person, and bring identification with you. American Express offices in the United States do not hold mail.