The basic idea is great but the presentation can be improved. As soon as the deck becomes the
envelope I reach into my back pocket and pull out a deck claiming that at the same moment the
envelope became the deck -- they magically switched places! Then I hand them the deck and ask them
to remove their card, which of course turns out to be missing. Then I say "The reason you can't
find your card is really quite simple. Tonight I wanted you to know what it really would be like if
your card truly vanished from the deck, and reappeared truly inside the small white envelope." I
slowly extract the signed card so they can clearly see it is really coming from inside the envelope.
I believe that's the way to do it if you want to maximize the magical impact. (Jeff Kaylor's UCCU or
TKO gimmicks, sold by Penguin, can be slightly modified to enable you to secretly ditch the first
deck when you turn around. That way you don't need a jacket and it's more angle-proof.}