This question came up after reading this page from WW2Today, discussing the "troublesome British POWs" who flaunted their rations in front of their guards and the German populace.
By what means of safe transport were care packages gotten to these POWs? Was there a "safe" port, or rail line, which by common agreement would not be bombed?
Germany signed the 1929 Geneva convention and allowed the International Red Cross to deliver rations to their Prisoners of War, provided these prisoners were from a nation that also signed that Geneva Convention. Since the Soviet Union did not sign, the Germans did not allow any Red Cross packages to go to their Russian POWs. The Soviet Union did not allow the Red Cross to help any of the POWs they held either.
Using the ports of Lisbon Portugal or Marseilles France, the International Committee of the Red Cross received 27 million + packages from the USA, 16.5 million from Canada and 20 million from Great Britain. They were shipped to Geneva Switzerland and distributed to various POW camps inside Germany. Pilfering was not a problem, until rather late in the war.
Source: http://www.cicr.org/eng/resources/documents/misc/history-world-war-2-overview-020205.htm
click the side bar to this overview for the details on prisoners of war.