Right. But we're talking nickels and dimes so what I'm thinking:
Take bare boards which are 1/32" or 1/16" thick, insert in a cardboard sandwich no more than 1/4" thick, insert in regular envelope. A few boards of the size we're dealing with will be less than 1 ounce. Apply 55 cents or Forever stamp plus 15 cent surcharge - there is even a non-machinable Forever butterfly stamp which costs 70 cents today. I like to mark the envelope non-machinable or give to USPS counter clerk who can mark envelope as such. Cross fingers that sorting station does not run envelope thru the high-speed sorting machine which might bend and crack the board.
If you have nothing better to do, look up youtube videos showing USPS sorting machines in action and you'll see why you want to pay the non-machinable surcharge! 
