When mailing a letter, you must clearly indicate the mailing and return addresses. The USPS also has specific guidelines for stamp placement -- in the upper right corner of the envelope.
Placement of Stamp
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The front of the envelope, or the side opposite the opening, needs three pieces of information on it. The return address should be in the top left corner, the mailing address should be written clearly in the center, and the stamp should be fitted securely to the top right corner of the envelope. Stamps should not overlap, and extra postage can be placed on the back of the envelope.
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Quality of Stamp
The USPS uses its own discretion when determining the condition a stamp can be in. A small corner tear or wrinkle is not a problem, but if a major area of the stamp is torn, missing or not visible, the postage may not be acceptable. Fix the stamp securely to the envelope to avoid tearing or falling off.
Exceptions
Big, misshapen packages make it difficult to use the top right corner for the stamp. The USPS allows some leeway as long as the stamps are in a logical place and grouped together.