Buyer claims package has not been shipped. Tracking says delivered. How would you respond to this?

Buyer: This order has not been shipped. We contacted ups and the package has not been picked up. We need this equipment immediately. I paid for expedited shipping, it’s been two weeks and nothing. No tracking information.

Seller: Hello, this order has been shipped and delivered. (Gives tracking # and date that it was delivered)

Buyer: This package was not received. no signature on file. no person listed. we do not have this. need replaced asap.

What would you do? Some things to note is I bought shipping through Amazon, it was an FBM order, I required signature and It was delivered before expected delivery date. Thanks.