Hi SF: You asked, "How would he indeed go about getting the package dusted for prints?"
I am not a crime lab expert, but I am aware that finger prints can be taken from paper objects. In the case of the famous Mr. D.C. Cooper, who hikacked a plane over Washington State, he had tried to hand a Flight Attendant some money from his stash. She refused, but the FBI later regretted her refusal because they would have had his finger prints on the money.
You also asked, "If there is no threat to his life, yet is suspicious, and, given the nature of his life these days, what CAN he do to find out where it originated?"
The postmark would at least show what state and town or postal center it was sent from. This, combined with a finger print from the package could yield some results, or at least have the information ready in the event some threatening material were sent. It is wise for SL to at least turn this stuff over to the police. He is in a high visibility situation that could lead to serious harassment or threats.
As I noted above, if there is no postmark (at least if after running a scan, the post office cannot determine that there was ever a postmark, then in all likelihood, it was deleviered by hand from a local crank. But, again, this is telling, and would at least allow for fingerprints. If the person concealed thier prints by using gloves, they still may have touched in in a way as to leave a minor amount of body moisture, and this could result in obtaining DNA information.
By documenting a trail as it first starts, this may set a pattern whereby the crank is eventually revealed, and this wold be important to establish in cases of harassment and/or threats. Using the USPS for harassment is a federal issue, and if I am not mistaken, may be a felony. - Amazing
