This story somewhat mirrors a similar RMA experience I had with Newegg a year or two ago.

I bought several high-end Cisco access points. One needed to be RMA'd, so I boxed it up and sent it back.

The problem was that I inadvertently used the box from one of the working APs. The remaining boxes were discarded.

Newegg refused to take the RMA because the serial number on the box's label didn't match the serial number of the unit inside. All of the original parts were there, still sealed in the original plastic. The RMA'd unit's serial number was on the original invoice. The sticker on the box bore the same part number and was in every way identical to the three others that were shipped to me, but the box's serial number was apparently worth $600 and the RMA could not be processed without it. I escalated my issue through several levels of management, all of whom were insistent that there was nothing they could do.

I posted about my experience on a couple of reviews sites, and within 24 hours Newegg relented and agreed to accept the RMA, conditional on me removing my negative reviews.