(3)
parcels exceeding 15 pounds but not exceeding 70 pounds in weight and not exceeding the maximum size allowed by the Postal Service for fourth class parcel post (known as “Standard Mail (B)”), including surface-type official mail, which are mailed at or addressed to any such Armed Forces post office where adequate surface transportation is not available.
Whenever adequate service by scheduled United States air carriers and military aircraft is not available to provide transportation of mail matter by air in accordance with this subsection, the transportation of such mail may be authorized by other than scheduled United States air carriers and military aircraft.