From time to time I see items, like the following, that are offered as being “dual franking” or “mixed franking” usage. They are NOT! The El Salvador stamps paid nothing. The first item is an unsealed printed matter cover that had a fee of 1c; the second a 1c post card rate. The third item should have been charged postage due as the stamps had been invalid since 1891 but it managed to slip through without being so. I have seen Christmas Seals used in a similar way and you know they didn’t pay any postal fee. All such items are mere curios.