One of the meanings of “express” is “explicit”: “Izaak claimed that his old boss had given him express permission to shop on eBay for fishing rods during work hours.” Some people feel the word should be “expressed,” and that form is not likely to get anyone into trouble; but if you use it you should not presume to correct others who stick with the traditional form: “express permission” (or orders, or mandate, or whatever).
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