What should be done when a buyer claims that they have not received their order?

First of all make sure about the delivery if it reached the specific spot or not, if not then your simply step to take would be to send another unit without charging them extras, so that they can keep on coming to buy from you.

If you are doing FBM then you need to investigate with the shipping company and check if the order has reached the spot or not, if it is missing simply send another unit to the buyer, if you are doing FBA then you can do case with Amazon or tell your buyer because it is Amazon who is handling the delivery.

If you are doing FBA, then this case will be handled by Amazon. In the FBM case, you have to contact the courier company that you are using to fulfill your order and then trace your order’s location. If your order is not delivered or is misplaced, you can contact the seller and ask them to send a new product to their order or return their payment if they have paid you for that order.