Hello @Purdy,
thank you for posting here. I am sorry to hear that you are experiencing issues with A-Z claims. First of all, it took me a while to understand that you are talking about orders from the same buyer.
This sounds like a well-calculated scam. I had similar things happen to me before, where a single buyer kept placing orders and opening A-Z claims alleging non-receipt despite clear evidence.
Unfortunately, in this case standard appeal routes won’t work. I would maybe look at your A-Z claim appeal wording to see if you submitted it properly. Have a look at my A-Z claim response guide thread: A-Z Claim Guide Thread
I would advise you to take the buyer to the Small Claims Court. This should be easy if both of you are in the UK.
Alternatively, you could lodge a claim with Royal Mail. However, it is really unlikely that two orders would get lost shipped into the same address.
Maybe others will have more ideas…