What item is prohibited from being mailed without permission from the postmaster?

The prohibition on mailing flammable substances without permission from the postmaster connects to safety and regulations concerning hazardous materials. The U.S. Postal Service has strict guidelines to ensure that potentially dangerous items do not pose a risk during transport. Flammable substances can create significant hazards not only to postal employees but also to the public if they were to ignite or otherwise cause a dangerous situation.

Non-narcotic medications, hypodermic needles, and prescription drugs are subject to regulations, but they can generally be mailed following specific guidelines and without extra permission from postal authorities. Non-narcotic medications are typically not restricted in terms of mailing, hypodermic needles can be sent safely when properly packaged according to state and federal regulations, and prescription drugs can be mailed by pharmacies and individuals following relevant laws. Thus, the correct answer emphasizes the unique risk posed by flammable substances and the need for oversight in their mailing.