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Mailing Standards of the United States Postal Service, International Mail Manual >
7 Treatment of Inbound Mail > 740 Irregular Mail > 742 Stamps Not Affixed
Some items of foreign origin do not bear postage stamps, but instead are marked “POSTAGE PAID,” “ON POSTAL SERVICE,” “SERVICE DES POSTES,” “TAXE PERCUE” or “TP,” or “PORT PAYE” or “PP,” followed by postmark. The marking On Her Majesty’s Service or O.H.M.S. is also sometimes used. Treat this mail as prepaid.
Some foreign post offices do not put postage stamps on parcels. All such parcels received must be regarded as prepaid.