In this situation in the past I have contacted the post office local telephone number and they have been able to retrieve the misdelivered package.
If the driver says it was delivered and the tracking number says it was delivered, then your problem isn't with USPS. You have a porch pirate or a neighbor who accidentally received the package. Either way, USPS won't be able to help as they only handle the shipping. Perhaps reach out to the seller and see if they can assist, otherwise it's like any other theft.
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