Tracking says delivered, buyer says not received

I have a customer who has told me that they didn't receive their item
even though tracking shows it was delivered a few days ago.

Customer says they think it was delivered to another door (perhaps next door)
and neighbors have in the past received their mail and never gave it to them, so
the customer is asking me to ship out another one (about a $40 USD value).

I told them that I would first have to open an inquiry with the shipping carrier
and see if they can find out if they can verify…

Tracking says delivered, buyer says not received