Here would be my verbatim reply... food for thought to all sellers.
Dear sir,
Sorry to hear of the problem that you report. I was completely unaware of the said discrepancy, and duly apologize for the oversight. As per my standard terms, I offer A 100% refund less shipping if not completely satisfied. In this case, however, whereby I overlooked the flaw, I will also cover the return shipping cost. Please advise as to where you would like the refund sent, so that same day return arrives, I will courier the refund to you (my dime). I believe that even with the flaw, this is a good deal on this case, and certain that I can sell it for equal to what you paid, so unfortunately, I cannot offer a reduction if you retain the case. Once again, I am very sorry about the oversight, and I look forward to your reply. I hope to work with you in the future and will do my level best to make our next transaction flawless.