Way to contradict yourself here. Yeah, it probably could be a federal violation to take postal packages with you, but certainly not leaving them. Why even consider something so dumb?
Are you saying you have to just leave it there forever?
I view it like this. If I throw a USPS package over my fence into your backyard, you would have every right to give it to the postman or put it in a mailbox. Could you be prosecuted for moving it, when it's on your property? Getting it back on track with the USPS doesn't seem dumb, it seems the easisest, simplest, right thing to do.