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You can't mail hazardous materials: explosives, flammable aerosols, loose lithium batteries, and alcoholic beverages are banned from USPS, with Canada Post enforcing similar restrictions. Live animals (with narrow exceptions), perishables, and anything illegal are also off limits. The short test: if it's flammable, pressurized, poisonous, or explosive, assume it's restricted.
Almost nothing, honestly. LetterAgent prints and mails letters only: your text on 8.5x11 paper, single-sided, in a standard double-window envelope. There are no inserts, no packages, no parcels, so hazardous materials never enter the picture. The one rule that still applies is that the letter's content has to be legal: the service won't print anything unlawful.
This guide covers the general rules for when you're mailing things yourself through USPS or Canada Post, since that's where the restrictions actually bite.
| Category | Examples | Why it's restricted |
|---|---|---|
| Explosives | Fireworks, ammunition, flares | Obvious: they can detonate in transit |
| Flammables | Aerosol cans, lighter fluid, oil-based paints | Fire risk, especially in aircraft cargo holds |
| Batteries | Loose lithium batteries and power banks | Thermal runaway fires; mostly limited to ground transport inside devices |
| Alcohol | Beer, wine, spirits | USPS bans alcoholic beverages outright; private carriers follow state laws |
| Live animals | Almost everything, with narrow exceptions | Welfare: day-old chicks and bees are the classic allowed exceptions |
| Perishables | Fresh food, anything that spoils | Health risk when it inevitably spoils in a hot truck |
Most of them are about airplanes. Mail travels in the cargo holds of passenger and cargo aircraft, where a fire can't be fought the way it can on the ground. Lithium battery fires in particular drove years of tightening rules: a damaged or loose lithium battery can enter thermal runaway, vent flammable gas, and ignite surrounding cargo. That's why loose batteries are heavily restricted while the same battery installed inside a phone or laptop is generally fine: the device protects it from the short circuits and punctures that start fires.
The alcohol ban is older and stranger: it's written into federal law for USPS specifically, which is why a private carrier can legally ship wine between states where it's allowed while the post office can't touch it at all.
Plenty of ordinary household items are restricted because of what's inside them. Perfume and cologne are alcohol-based, so they count as flammable. Nail polish and nail polish remover are flammable. Bleach is corrosive. Dry ice sublimates into carbon dioxide gas, which can displace air in a sealed cargo hold, so it has strict quantity and labeling rules. Even some cleaning products and spray starch fall under the rules.
The pattern: if the product has a flammable, corrosive, or pressurized warning on its label, the post office probably restricts it too. When in doubt, check the USPS Publication 52 (Hazardous, Restricted, and Perishable Mail) or ask at the counter before you seal the box.
Ready to mail it? Send your letter with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it's printed and mailed.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
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