SF sent me a short message saying that the courier had been left in our bookstore downstairs, so I went to look for it.

First of all, you should contact the seller and ask the seller to contact the courier for you to see if the delivery is successful. If the delivery is successful, you didn't sign for it. There must be no signature on the courier, so this responsibility is the responsibility of the courier. Don't confirm the receipt if you lose it. Let the seller negotiate with the courier company.