Has the estimated delivery date passed?
Did you send the item with tracking?
If the item is overdue your buyer is entitled to open an "Item not received" case, and if you have not got a tracking number to prove delivery you will have to refund.
If you have a tracking number enter it into the sale in your seller hub, and if it shows delivered you will not have to refund.
It is worth remembering that Royal Mail parcels have a reference number on your proof of posting. Entering this number in RM Track and Trace will show up if it has been delivered or not, so enter that number into your sale.
