Package claims was empty when delivered

Hi

I am looking for a bit of advice. A buyer contacted me to say that he received an empty package. He also stated the package had not been tampered with. At the same time he opened a A to Z claim before I even had a chance to respond. I checked all the details and have a receipt from the Post Office stating the package weighed 300g and £2.68 was paid in postage. I provided this information to Amazon on the A to Z case. I replied back to the customer attaching the receipt that the package was not empty when we posted it and asked if the packaging may have been opened or damaged. The customer confirmed again that the package had not been opened or damaged. I emailed back to say if the packaging had not been damaged enough for the contents to fall out then it may be the case there has been a possible theft within the Royal Mail and will open a case with them. Immediately my customer responded saying the packaging had been damaged. The buyer demands a refund and is not interested in any replacement. Amazon closed the A to Z case and informed that the customer will not be refunded. My customer appealed and again Amazon decided not to refund. Now I am receiving 6-8 emails per day demanding a refund from the buyer. I am finding it hard to believe that the package was empty since his story has changed. I also thought that once Amazon made a decision it was final. Should I respond back to his demands?