Heller II is already in the courts (types of weapons) Heller 2.1 (incorporation) is @ the SCTOUS level already. You are correct, that Heller 3.0 will be “right to bear” arms.
SCOTUS has already ruled that you (I believe only by dicta though) that you cannot “prohibit” individuals from open carry, because 2A IS your permit to do this, however, you can discriminate for concealed permits, only if you Constitutionally allow open carry.
This has been partially adjudicated, at least here in Texas (5th Circuit) IIRC.
I don't think the postal service could enforce this 'regulation' in the 5th circuit.