The carrier's tracking indicates that the package has been delivered to the recipient, but your customer tells you that they have not received it: TPOP does not cover the shipment of a new package.
The carrier's tracking is authentic: if the package is marked "delivered" there is no recourse.
Very often the package will have been dropped off at a neighbor's house or the customer does not have a standardized box:
In this case, your customer should be advised to go to the nearest post office (if the carrier is the Post Office), with his tracking number and an ID to retrieve his order.
If the mailbox is not standardized or the package cannot be deposited there (because it contains too many products), it also happens that the postman/deliveryman deposits it at the post office.
If the customer tells you that he has not been able to get his parcel, the decision is yours: you can tell him that once the parcel is indicated as "delivered", you are not able to send him a new order or you can decide to place a new order manually at your expense.
Il pacco è contrassegnato come "consegnato" ma il mio cliente non ha ricevuto l'ordine. The package is indicated as "delivered" but my customer has not received his order. El paquete está marcado como "entregado" pero mi cliente no ha recibido el pedido. Das Paket wird als "geliefert" angezeigt aber mein Kunde hat seine Bestellung nicht erhalten. The package is marked as "frozen", but my customer has no idea what to expect.