Seller suport/atoz terrible

Anyone else experienced this?

We sent out 5 x £300 items to a client in two shipments, 2 items in one box, 3 in another box, client received first box of 2 items. They then contacted us asking where the rest of items are- we told them they are on route but due to delays in postal etc please be patient. Within an hour of our response they logged an atoz and the customer is automatically refunded. The 2nd parcel was subsequently delivered the following day.( it was delivered within the delivery window) We then appealed this to which amazon email us saying please send weight dims etc to prove the item was inside the box as the customer claims the box was empty- we have 72 hours to comply. Within 6 hours of this response from amazon we were then informed the decision has been upheld even though we had not responded as we had 72 hours! We now can’t send the weight dims or anything else to appeal this. Subsequently after messgaing the customer they admit they have the items and don’t want a refund. I raised this with SS who say we have to coverse with the customer directly to seek our money or raise an SAFE-T (which won’t work because it was shipped by us not ‘buy shipping’. The customer is now not responding to our messages. I mean wtf we are out of pocket of £1500 of goods and since when do WE as sellers seek mon ies from customers directly! I have complained to MD but nothing. Any help here would be appreciated