I plan on putting the postage and label in a separate envelope. For the return of your novel, provide a envelope big enough to hold the box your manuscript is in. Put an address label on the envelope, along with the postage. If you sent your manuscript in one of those heavy-duty manuscript mailing boxes, you can include a return label and postage inside. The publisher will tape the box shut, and apply the label and new postage.
Nobody wraps manuscript boxes.
Don't send the manuscript in a box that is twenty times the size of the manuscript. And make sure the box is easy to open. If you want the editor to use the box to return the manuscript, make sure the box is also easy to seal. Bubble-pak envelopes are a good choice if you use an envelope. Tyvek envelopes seem to result in very battered manuscripts which are harder to page through.