I pack flat items in a bag and board, sandwich between cardboard, and then in a plain manila envelope or polyjacket (plastic) envelope. I have actually found that shipping flats in bubble mailers actually encourages bending, and it is very hit and miss whether carriers pay attention to anything written on the package other than the address. Basically, the more physically difficult it is to bend, and the more it resembles shipping a board, the better.
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