The aromatherapy store near me sells mists and a few years ago they had a couple of xmas ones....last year I was going to get them and mist them into the envelopes but then couldn't find the ones I had liked (out of stock). I was going to spray in the envelope and let it dry a day before putting card in there...dampness can make your cards warp and if you spray right on the card it may stain or damage the colouring. I think this is the one I'll get this year:
http://www.saje.ca/product_p/2000.htm