RM dont class anything lost for 15 working days. Ask the buyer to check with their local delivery office to see if it is there, as the postie doesnt always leave a calling card.
Did you get the cop (cert of postage) stamped at the PO, or ask them for a receipt when you sent the item, as it may be difficult to claim without it
You would neded to refund the buyer in full if it doesnt turn up and claim yourself