In the US, if a package is correctly addressed to you but you did not order it (unsolicited merchandise), you may legally keep it as a promotional gift. If a package is addressed to another person and simply misdelivered, you're ethically and legally obligated to make an attempt to return it.
The rule allowing recipients to keep unsolicited merchandise in the United States was created to combat deceptive marketing practices. In the mid-20th century, some companies began sending unordered goods—such as books, jewelry, or household items—to consumers and then demanding payment, often implying that failure to return the items constituted acceptance of a purchase. This tactic exploited people who might feel obligated to pay or were unaware that they could refuse the merchandise.
As for the CSR having difficulty understanding your situation, most low level reps are given little training with a list of common problems and remedies. If your problem isn't listed, they're not equipped to respond.