The Federal death penalty is only used for crimes involving death, and they don't use the post office for capital trial information, scheduling, and/or notifications.
There is a fundamental difference between policing and military action. Applying civilian legal standards to military situations is a bad idea because it inevitably results in watering down ordinary criminal protections to accommodate the needs of military situations.
Al Awalki was in a foreign country--where U.S. criminal laws presumptively do not apply--organizing armed action by foreigners against the United States. It's an archetypal act of war, rather than a crime.
But even so, why ask me? You're a lawyer, you know how to provide extraordinary evidence to support extraordinary claims relevant to what is by no means a settled question.