Mailing a firearm falls under the "except for official purposes".
Official purposes refers to the presence of the firearm on postal property, not the presence of the person themselves.
For instance, a cop wears his duty firearm "for official purposes". A military person wears a firearm issued to them by the government "for official purposes" (although they probably wouldn't be at the post office!) A person mailing a gun has the gun itself there for the "official purpose" of mailing it.
Now, while Joe Citizen may be at the Post Office conducting postal business, the gun that he is wearing is for personal protection and is NOT for "official purposes" and, thus, illegal to possess, carry, or store on postal property.
Official purposes refers to the presence of the firearm on postal property, not the presence of the person themselves.
For instance, a cop wears his duty firearm "for official purposes". A military person wears a firearm issued to them by the government "for official purposes" (although they probably wouldn't be at the post office!) A person mailing a gun has the gun itself there for the "official purpose" of mailing it.
Now, while Joe Citizen may be at the Post Office conducting postal business, the gun that he is wearing is for personal protection and is NOT for "official purposes" and, thus, illegal to possess, carry, or store on postal property.