Expired license plates can be returned at a DMV office or by mail with an explanation since insurance cancellations are not covered by the extension.
You can return the plate to a local DMV tag office or mail* it to:
DMV Vehicle Registration Section
3148 Mail Service Center
1100 New Bern Avenue
Raleigh, N.C. 27699-3148
*Please include a brief note about why it is being returned, and you should request that a receipt for the plate be sent to you.