Phase 4: Card Visually Jumps to the Top Again + Sticker
A sticker is then placed on the back of the selected, signed card and signed with the spectator's initials. The card is yet again placed in the middle of the deck. With a simple flick of the deck, visually a sticker appears on the back of the deck, and as you guessed it...it's the spectator's signed selection.
Sleights Used: Triple Lift...DL...Pass...Marlo Tilt + Ego Change
If this is done with a pass you should have them sign the back of the card instead. It removes the possibility from their minds of you changing the sticker from one card to another.
I'd be tempted to implement the bent card jump in your routine. It might be overkill, but it fits in well with the theme of constantly making the conditions more stringent.
If you're not familiar with the move, you do a DL showing their card. You put a visible bend across the middle of the two cards. (shortways, not longways) You replace them on top of the deck, holding the selected card flat with the deck in an elevated dealing position (your thumb and middle finger hold near the bend to keep it flat). You insert the other card into the deck and show that it's bent inside the deck. You flatten out the bend and then do your magical sign. Release with your thumb and middle finger and the card appears to visually jump to the top of the deck.
You can then do a pressure fan to the entire deck to remove the bend from the dummy card that was put into the middle of the deck.