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IMM Issue 34 - International Mail Manual >
2 Conditions for Mailing > 250 Postcards and Postal Cards
Postcards and postal cards consist of single cards sent without a wrapper or envelope. Folded (double) cards must be mailed in envelopes at the First-Class Mail International rate of postage.
Reply–paid cards are not accepted in international mail, except as provided in 132.2.
Postcards must be made of cardboard or paper that meets the material and color specifications in 142.
Privately manufactured postcards, except picture postcards, must bear the heading Postcard.