In regards to the postal worker militia. It is true that the laws of war does recognize that a soldier forfeits his rights as a POW if he is not wearing a uniform. However as Sid pointed out there is a great difference between SS units putting on civilian clothes and postal workers taking up arms against an invading force.
The soldier is of course obliged to carry arms openly and wearing a uniform as is stipulated in article 1.
Article 1.
The laws, rights, and duties of war apply not only to armies, but also to militia and volunteer corps fulfilling the following conditions:
To be commanded by a person responsible for his subordinates;
To have a fixed distinctive emblem recognizable at a distance;
To carry arms openly; and
To conduct their operations in accordance with the laws and customs of war.
In countries where militia or volunteer corps constitute the army, or form part of it, they are included under the denomination "army."
However the Article 2 also recognises that a nation during the first initial fase of an invasion can be given the right of creating makeshift defense corps without wearing the uniform as long as these carry arms openly and respect the laws of war:
Art. 2.
The inhabitants of a territory which has not been occupied, who, on the approach of the enemy, spontaneously take up arms to resist the invading troops without having had time to organize themselves in accordance with Article 1, shall be regarded as belligerents if they carry arms openly and if they respect the laws and customs of war.
So if the german forces apprehended polish fighters who had carried their arms openly they are to treat them as POW's as these makeshift units would fall into the category of article 2.
Scorzeny units during the battle of the bulge are illegal combatants and does not fall into the same category as the above mentioned poles.