Unless prohibited by the country of destination, merchandise or other articles within the applicable weight and size limits may also be mailed at the letter rate of postage. The weight limit for letters and letter packages is four pounds. Aerogrammes are air letter sheets which can be folded into the form of an envelope and sealed. Tape or stickers must not be used to seal aerogrammes. Enclosures are not permitted in aerogrammes.
Postcards are single cards sent without a wrapper or an envelope.
These measurements apply to all countries. AO mail includes regular printed matter, books and sheet music, matter for the blind, small packets, and publishers' periodicals. Dimension restrictions for AO mail are the same as for LC mail. Printed matter means paper on which words, letters, characters, figures, or images, or any combination of them not having the character of a bill or statement of account, or of actual or personal correspondence, have been reproduced by any process other than handwriting or typewriting. Included in this category are regular printed matter, books and sheet music, and publishers' periodicals.