An incredible gimmick with multiple possibilities.
Routine 1: Change
Make the spectator choose a card that he will sign and put it back in the deck. Then snap your finger and a card goes up on the top of the deck but bad luck it is not the spectator’s card. So take the card and put it on the of the deck and in a second the card turns into that of the spectator which you can immediately give the card for examination and as a souvenir.
Routine 2: Vanish
Make the spectator choose a card that you put on the top of the deck, face up and sticker a sticker on the card. In one second, the sticker disappears from the card.
Routine 3: Transpo
Have the spectator choose two cards place on on the table, face down and the other card on top of your deck face up. In a second the two cards swap in a second, immediately deal the two cards at the exam.