can you even do certified mail with P.O. Box as address (e.g. Nebraska Center)?
I had once asked the same question at the Post Office itself (while facing a similar dilemma) and as per the answer given to me you can do a Return Receipt / Certified Mail for a P.O Box that is with the USPS (and not a pvt. company like Mail Boxes Etc). NSC would be a USPS P.O. Box for sure. So the answer is YES.