Matador....yes, just paper without anything written on it even newspaper used for packing. USPS has started checking more of the media mail packages (it says they are subject to being opened for verification of rules compliance) and are considering anything added to the books even a written thank you note as advertisement and disallowing the media mail rates. I've actually only had one that this happened to, but since it was a 30 pound box of books it made me more careful to keep it strictly books inside.
I also sell vintage magazines which are now not considered media since they contain ads even if they are 50 years old. I got those turned down at the post office before I closed the box. It might have made it through without being spot checked if I had it sealed before bringing it in, but it's not worth the gamble (21 lbs that time). I'm using FedEx on all my heavier packages now but it's a 50 mile round trip (or a pick up fee) so I have to get several packages to send at once to justify the trip cost.
USPS flat rate envelopes are good for mailing books also if the buyer doesn't mind that shipping cost.
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