Yup done it quite a few times and have also been the person asking for a card to be sent like that....mainly when I have bought a $1.00 - $3.00 card and the postage was $3.00....simply asked the seller to place the card in a penny sleeve & toploader & tape it at the top. Then was only charged $0.50c to $1.00 thus paying 1/2 the amount I would have. Might seem like pennies to some but at the end of the day it all adds up.
I do also recycle bubble mailers so i agree with Gemini, I mainly like to do this becuse I am a bit of a "greenie" when it comes to recycling