I know, this is the section that is frequently posted and referred to. But, I have something else to add.
(a) Except as provided in subsection (d), whoever knowingly
possesses or causes to be present a firearm or other dangerous weapon in
a Federal facility (other than a Federal court facility), or attempts to
do so, shall be fined under this title or imprisoned not more than 1
year, or both.
...
(d) Subsection (a) shall not apply to--
...
(3) the lawful carrying of firearms or other dangerous weapons
in a Federal facility incident to hunting or other lawful purposes.
To me, this sounds as if...you had been hunting, and walked into a post office, with a firearm that you were using for hunting purposed, it is perfectly OK. Keep in mind, as I understand it, Delaware has a handgun hunting season. Now, I am not a hunter, but this is what I have been told. So, walking into a post office, with a handgun, after hunting with that same handgun, should be perfectly legal.
Now for the other part of the wording..."other lawful purposes" means, according to the dictionary, "other reasons allowed by law". So, it could certainly be translated as..."legitimately carrying a firearm for other reasons allowed by law". In Delaware, we are legally allowed to open carry...so I would interpret this as allowing concealed and open carrying inside a post office...the need for CCW would be mute.
Also, keep in mind...no sign = no conviction
I just thought I would throw all of this out there for added discussion, do not take this as legal advice.
As everyone else has done, I, too will add my disclaimer...I AM NOT A LAWYER...if you follow these interpretations, you do so at your own risk.