Do it anytime during your show or as a running gag. Several presentations are included. You provide the case, we provide the balls. See the next item. You provide the case, we provide the balls.
Endless effects are possible.
Ask him to fold it and place it into the box for safe keeping. The box is hung around the spectator's neck, in full view. You concentrate, then write a serial number on a piece of paper. The box can be checked by the spectator, even after the switch is completed. The switch is invisible, and is done without you touching the box.