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.
Kind of a logic to it if you think yourself to be sovereign.
From https://pe.usps.com/IMM_Archive/HTML/IMM_Archive_20060108/imm/immc7_004.html
742 Stamps Not Affixed
742.1 Marking
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.
Kind of a logic to it if you think yourself to be sovereign.