Hello @wannacry,
thank you for joining the forum. You may be aware that I used to be a leader in advising others how to defend A-Z claims. At that time, I managed to either win or have Amazon Funded the majority of the claims. I created a whole guide with templates advising how to formulate your A-Z claim responses, which you can find here:
However, over the past half a decade or so, Amazon changed their manner of handling them drastically. I don’t think I had any A-Z claim either denied or Amazon Funded over the past 2 years. The company doesn’t care if you have full tracking / signature proof of delivery.
They will now fund them from your account, if the buyer simply insists in correspondence, that the order was not received.
If you are UK, based, at least you can take the buyers to the Small Claims Court, Many have been successful, as also reported by @Purdy or @AmazonUKForumRefugee.