The U.S. Postal Service bans the mailing of cigarettes, alcoholic beverages, perfume, plant materials and potentially hazardous or explosive materials to any international destination, as of 2015. This includes postage to Canada.
Potentially hazardous or explosive materials include dry ice, ammunition, aerosols, gasoline and nail polish. This list is not exhaustive and is subject to change. Shippers should check with the U.S. Postal Service and Canada Post to ensure that all current regulations are met before shipping a package to Canada.
Canada prohibits the mailing of illegal materials, products or substances that may injure postal workers or damage postal equipment and sexually explicit material that is not properly labeled as adult content. Specific examples of banned materials include military munitions, firearms, tobacco and live animals. These restrictions apply no matter what country the items are mailed from.
Some items that are not prohibited are subject to restrictions. Lithium batteries are only allowable when installed in the equipment in which they are used. Lithium batteries that are not installed in equipment cannot be shipped internationally, even if they are packaged alongside the device. Medicines and drugs must be legal for import in Canada but are not prohibited by the U.S. Postal Service, so long as this requirement is met.