Best/Safest Way To Send Sheets?

I will be mailing some sheets. 8.5 X 11.

They'll of course be in plastic sheet protectors. I could send them in plain, un-bubble-wrap manila envelopes. Or perhaps I'd send them USPS Priority mail in the stiffer cardboard envelopes they use. In either case, should I put at least some thin cardboard on each side? Maybe even thicker corrugated cardboard (like the backing of a picture frame?)

I just want to make sure they arrive flat and dry.

Any ideas or experiences welcome. Thanks.

yes thick cardboard if you can and use the protectors also leave some space on each side and the corners because they can take a hit.

I use Corex sheets, the same or similar to the Corrugated Cardboard but in plastic. Many stamp dealers are using this, at least UK stamp dealers.
I was lucky enough to find about 100 sheets or more a couple of years ago and they will last me forever !
When using this, or the corrugated cardboard, cut the pieces you need in opposite directions so the lines/lanes of the corruagted cardboard lie in different directions on either side of the sheet.
This type of cardboard can bend, but it is much safer if they are cut both ways, one for each side of the sheet.

Good luck.

Londonbus1

marckx, I recommend following the advice of spock and londonbus. I recently received a 8½ X 11 envelope that had been somewhat bent but the two sheets of cardboard prevented the envelope from being sharply creased. The stamps on the sheets arrived in very good shape.