Refund to customer by Amazon

Kens Books: I received order from customer on Thursday 9/9/21 I posted books as I always have done on same day on receipt of order. On the 2/10/21 customer emails me requesting a refund, I in turn phone the Royal Mail Delivery Office closest to his address, I supply royal mail with customer confirmation reference number , the reply from royal mail was ''this parcel was delivered on Saturday 2/9/21. Two days after I posted, which is usual standard delivery time. I have supplied seller support and put in a A-Z claim. I have put this on this Forum Site 1/ Maybe I am using you all as a listening post 2/ Have any of you been in this situation and what was the outcome .