Customer refunded £160 with no message, no return, no a-z claim. Safe-T claim denied because I don't use "Buy Postage" on Amazon

I'm quite shocked.

I used tracked and signed postage with royal mail's business portal. I paste the tracking numbers in.

Out of the blue I'm at minus £160 with no clue why, customer support tells me the parcel was not delievered. No messages, no a-z claim.

I send the track and trace link that was attached to the order, it's there, that's their front doorstep and their signature, could I have my money back please?

Get told no and to make a safe-t claim.

My claim was just denied because they only protect people that buy postage through amazon.

That's unbelievably unfair, has anyone been through this before? What can I do?