The advice from Couness is very good.

You should probably get onto CS and explain the situation with particular reference to the incorrect customer details and lack of communication.

I would personally use online chat as the CS reps seem a bit more knowledgeable and you'll get a transcript of the convo for file and future reference.

Make sure you mention that you've sent them a return satchel (I presume you've recorded it's tracking number?).

You MUST, as said before, make contact through the resolution centre (quote the return satchel number when you do this).