Do you mean they are saying that they did not receive the item? Did you purchase Delivery (or Signature) Confirmation with your shipping?
If you have some kind of delivery confirmation or tracking -- what does that say? If it says the item was delivered and you delivered to the address that was on the account when the buyer paid for it, then you have no further obligation.
If you did not purchase delivery confirmation or have tracking, you will be reimbursing the buyer in full.
“Let’s work the problem, people. Let's not make things worse by guessing.”
-Gene Kranz, Mission Control Director (played by Ed Harris), Apollo 13
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