This is sometimes done with and without a subpoena through law enforcement agencies. Since this is both an FBI and postal issue (USPS does many Internet services), I would direct your questions to your local FBI agent and Postal Inspector. They should be able to walk you through the process or provide better focus.
In the past when I did consultant work, I was required to do the same but with MS, AOL and others. We did it all through subpoena. Per the Patriot Act you may not need a subpoena based on the damage and proof you can provide. Again, this is law enforcement and should be handled as such.
For more information on this topic, visit these other SearchSecurity.com resources:
Executive Security Briefing: The Patriot Act and Carnivore: Reasons for concern?
Best Web Links: Law, Public Policy & Standards
This was first published in February 2003