The Postal Service’s definition of a hazardous material includes many common household and consumer products. These items may not be hazardous during normal use or storage in your home but can present a significant hazard when placed in the mail due to vibration, temperature changes, and variations in atmospheric pressure.
Some examples of commonly-used items restricted or considered hazardous under USPS regulations include:
Perfumes
Nail polish
Flea collars or flea sprays
Aerosols
Bleach
Pool chemicals
Paints
Matches
Batteries
Fuels or gasoline
Airbags
Dry ice
Mercury thermometers
Cleaning supplies
Items previously containing fuel
Glues
Fireworks
Other items, such as alcoholic beverages (beer, wine, liquor), are not considered hazardous but are prohibited and boxes displaying such markings are also prohibited.
Tobacco products are presently eligible to be entered into the mail. The USPS is concerned about cigarettes and smokeless tobacco being sent to minors; they are also concerned sales of cigarettes over the Internet frequently violate the Jenkins Act, which requires the cigarette seller to send reports to the destination-state's tax authorities to facilitate the collection of state taxes. Such sales may also run afoul of state tax and public health codes. Sending cigars to an adult as a gift through the USPS is perfectly legal.