The Emperor's New Groove : Why would Yzma mail the box to herself?

With her original plan to kill Kuzco, she wanted to turn him into a flea then put the flea in a box, put that box inside of another box then put that box inside of another box, mail the box to herself, and she then smashes him with a hammer. However, why not just smash the box right there? Things get lost in the mail, so there is no promise that she would get it back.

Because it's a hilarious monologue in a hilarious scene !