Different couriers and countries have different things that they will not accept for being shipped through parcel sevices. Ensure that you know whether you can ship the items to their destination BEFORE you package them and preferably, before you BUY them.
Trying to send things that you are not allowed can minimally result in the loss of your shipment to charged by the courier for expenses and costs for having to deal with the effects of shipping something to being charged and prosecuted under the law.
The following are considered Dangerous Goods and are not allowed to be sent via courier except under highly controlled conditions
Chemicals : are usually prohibited in most forms, although some chemicals may be allowed or allowed in certain forms (liquid, powder, solid, etc).
Explosives : Including signal flares, ammunition, fireworks, detonators, black powder, cartridges, dynamite, flash powder, grenades, nitroglycerin, rocket motors, toy caps, starting pistol caps, explosive fuses and igniters, etc.
Gases : Aerosol containers, butane, propane, carbon dioxide cartridges, cigarette lighters, fire extinguishers, compressed gas cylinders (various contents), cryogenic liquids, mustard gas, etc.
Flammable Liquids : acetone, adhesives (glue), alcohols (butanol, methanol, ethanol, etc.), camping fuel, aftershave lotion, paint stripper, gasoline, gasoline additives, paints, enamels, lacquers, perfumes, solvents, thinners, turpentine, naptha, petroleum distillates, etc.
Flammable Solids : calcium, oil-impregnated fabrics, fire starters, fish meal, lithium batteries, magnesium, matches, mothballs, napthalene, nitrocellulose products, oily cotton waste, fuel tablets, phosphorous, sodium, etc.
Human Remains : Human remains in any form are usually not allowed for transport.
Oxidizers & Organic Peroxides : adhesives, bleaching powder, disinfectants, ammonium nitrate fertilizer, fiberglass repair kits, hair & textile dyes, nitrates, organic peroxides, oxygen pellets, hydrogen peroxide, oxygen, etc.
Toxic & Infectious Substances : antiknock compounds, arsenic, carbon tetrachloride, chloroform, disinfectants, drugs, dyes, fungicides, hepatitis, HIV, infectious biological products, infectious blood, wood preservatives, photographic chemicals, herbicides, pesticides, insecticides.
Radioactive Materials : medical isotopes, measuring instruments, etc.
Corrosives : acids, battery acid, caustic soda, disinfectants, drain openers, dyes, etching fluid, formaldehyde, household bleach, paint or varnish remover, potassium hydroxide, sodium hydroxide rust remover, soda lime, cleaners, fire extinguishers, lye, oven cleaners, etc.
Other Dangerous Goods : asbestos, battery-powered devices with the battery installed, dry ice, hazardous wastes, self-inflating life preservers, air bags, strong magnets, gas-filled shock absorbers, etc.
Most couriers prohibit the transport of weapons such as firearms or firearm parts or ammunition except under special conditions including a waybill that requires signatures after each hand-off and special shipping cases. Not all couriers are able to offer the transport of weapons.
Foodstuffs may have restrictions on shipping, including no sending of frozen items, no use of dry ice or other limits. Food items being sent into the USA require prior notice and may be subject to additional paperwork in order for them to be sent across the border.
Most Muslim countries have bans on items that may be offensive to the faith of Islam, such as pictures of women semi- or unclothed, pork products or anything derogatory to the Koran, the faith, religious leaders, etc. Inclusion of one such item in the shipment may result in the loss of the entire shipment and there may be consequences for the intended receiver of such a shipment.
Cash or cash equivalents (stocks, bonds, etc.) are not insurable through a declared value with the courier. If you send cash or cash equivalent through the courier and it goes missing, there is zero liability with the courier. You must insure through a 3rd party.
Drugs (Prescription and non-prescription): Some countries prohibit the importation of drugs while others highly regulate the importation of drugs. If you are allowed to send drugs to a particular country you should be prepared to include a complete medical description of the drug along with an original prescription to the person receiving the drug, along with cost and country of manufacture. Certain drugs may not be allowed while other drugs may be allowed.
You, as the sender of the item are responsible for the proper description and disclosure of all items in your shipment. Failure to fully disclose such items could land you in trouble with the law and be personally responsible financially for any loss suffered by the courier, their carrier and anyone else that your shipment affected.
Also be aware that ALL shipments (envelopes & parcels) are subject to inspection by the customs agents in the country you are sending to. This means that items may be opened up to be looked at based upon your description or may be randomly opened for inspection as a general look to ensure that items are correctly marked on the commercial invoice.
Some countries customs agents are more honest than others, which means that in some countries, items may go missing from packages before they are delivered to the receiver. Theft happens, whether from dishonest employees of the courier or people that are hired to assist with other aspects of the delivery of your package or government inspectors who look at your package . If your item is of some value, ensure you have proper insurance or declared value protection on your item before sending it away.