Royal Mail untracked not received - just refund?

I’ve just had my first INR in a very long time, for a book posted first class large letter. I had a message at the time from the buyer asking why I was sending it as he’d ordered from world of books (??), I replied it had gone first class and should arrive in a day or so. No reply or query for 18 days, then exactly the day after the deliver by date I get “Item not received, please refund”.
Odour of rodent, but what can I do? no clue Googling the buyer. Guess I’ll have to refund and attempt a claim via RM - have the COP. value about £20.