Never received order, but tracking says they did?

I get what seems like an unusually high amount of people saying they never got their order, but tracking shows they received it. I always respond saying it was sent via US postal service, and to check their mailbox or see if someone brought the package inside. Half the time, they didn’t realize this, and they find the order. The other half, they can’t find the order.

They paid for the order, so they shouldn’t be liable. I shipped the order, and there’s proof it arrived, so I shouldn’t be liable either. However, they never got it, so what do we do?

Thanks!