If you ship printed material via Media Mail, do not use pieces of Priority Mail or Express Mail boxes as stiffeners. The USPS has the right to open Media Mail for inspection, and even if the contents qualify, if you use Priority or Express materials (even though those materials are obviously used) they will assess postage due just as if the parcel was shipped via the Priority or Express service. A year or so ago there was an article in the APS monthly magazine about this.
Please do not use tape, staples or paper clips to hold items together or to seal edges. Stamps shift in shipment and if they come into contact with a piece of tape, even just the edge of the tape, it can damage the stamps. As to staples and paper clips, they also can damage stamps even if you think your use is clear of the stamps themselves (not to mention the hand of the unsuspecting buyer when he slides his hand into the package to retrieve the stamps).
Please seal against moisture. I live in a very wet and humid state, and in my rural area the carriers often leave oversized packages outside my door. UPS and DHL wrap their deliveries in plastic bags to prevent damage, the USPS does not.
Thanks for listening.