USPS will only allow a PWE to be 1/4 inch thick. If you put a piece of cardboard in there you are going to exceed that and if so shipping will be charged as a package. I have had to pay postage due many times from a seller who tried to put cardboard in an envelope and mail it with just a stamp.
A PWE is principally used to mail letters, and they are sorted through mechanical feeders. USPS has a special 85 cent stamp you can use to have the PWE hand sorted. My local post office advised that you should also mark the envelope NON MACHINABLE in red on the left side.
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