Look before you reply
Open the tracking and the order first. Know whether it is in transit, delayed, marked delivered or stuck. A reply that asks for details you already hold wastes the customer's time.
Say what you found
Tell them the last scan and where it was, in plain words. If the carrier shows it as delivered but the customer says no, take that seriously and ask them to check with neighbours and the building before you make a claim.
Own it even when the carrier is at fault
The customer bought from you, not the carrier. Do not point at the courier and stop there. Say what you will do about it.
Offer a clear fix
Where the parcel is truly lost, offer a replacement or a refund and let the customer choose. Do it without asking for proof beyond what you need. Match the offer to your refund policy and say how long it takes.
Open the carrier claim yourself
Raise the claim with the carrier in the background so the customer does not have to. Record the claim number on the ticket.
Watch for patterns
Tag lost-parcel tickets by carrier and region. If one route keeps failing, change the service or add tracking and insurance for that route.