From the USPS web site....
Many household items can’t go in the mail.
Restricted materials come in a wide variety of forms and can be chemical, biological, radioactive, or a combination of factors.
Many common household and consumer products cannot be mailed through the US Postal Service®. These materials may not be dangerous on your shelf at home, but they may become a hazard when shaken or when exposed to temperature or pressure changes. Additional rules and restrictions apply for items being mailed internationally.
Examples of household products that may be mailable but have specific restrictions…
Aerosols
Lithium Batteries
Nail Polish
Perfumes containing alcohol
Pool Chemicals
Paints
Matches
Certain Glues
Examples of non-mailable items…
Air bags
Ammunition
Explosives
Gasoline
Browse Domestic Mail Manual (DMM®) 601.10: Hazardous Materials ›
Browse Publication 52 -- Hazardous, Restricted, and Perishable Mail ›
Also
There are rules and restrictions for what can and can’t be mailed.
This list is not exhaustive, but in most cases, you can't send…
Alcohol — Alcoholic beverages (beer, wine, or liquor) are restricted and can’t go in the mail. If you’re reusing a box displaying alcohol brands, all logos and labels need to be completely removed.
Perfume − Perfume containing alcohol is prohibited on air transportation, and can only be shipped domestically via surface transportation. Perfume containing alcohol is prohibited in international shipments.
Prescription Drugs – Prescription medications can only be mailed by Drug Enforcement Administration (DEA) registered entities. Similar regulations may apply to over-the-counter medications.
Cigarettes & Smokeless Tobacco – With few exceptions, cigarettes and smokeless tobacco shipments cannot be mailed.
See tobacco restrictions and guidelines ›
Firearms – Only licensed manufacturers and dealers can mail or receive handguns. And even though unloaded rifles and shotguns are mailable, mailers must comply with all applicable regulations.
Lets remember this is a branch of the US Government and they don't want us to HAVE Ammo anyway.