What would you do with this mystery package?

I ordered something online, and it arrived today. When I opened the package, there was another package inside it.

It contained a SF 49ers autographed football card of Cody Pickett. I flipped the envelope, stumped, and found a USPS label attached to the other side. I believe it was included in my shipment by accident. The tracking number has already been entered into USPS’ database.

Would you report this to the retailer, just send it on its way, or keep it? (I’m gonna just send it…)