There is also a red sticker with diagonal stripes in my return box & no instructions as to what to do with it. Any ideas?

I am trading in my old iPhone. I received a box to return it and followed the instructions from Apple help to prepare it for shipping.

There is one item included in the box that isn’t mentioned in the instructions. It is about a 3 inch sticker with red diagonal stripes. Where is it supposed to be put on the box?