Shipping to an another country means exporting. There are different rules and definitions with regards to firearms and their parts.
Within the U.S a barrel, or a slide would only be considered a firearm part and therefore there are no regulations as to shipping them. As far as exporting is concerned, in addition to frames, parts such as barrels, cylinders, slides and complete breech mechanisms would be considered firearms.
These require a license to export from the department of defense.
The department of defensive trade controls handles this, their website:
www.pmdtc.org
Do a search on google for Firearm exporters, they are usually FFLs who also handle these type of things. Usually an import license, and purchase order are required from the buyer. It may take anywhere from 2 weeks to 3 months for them to clear the export license.
The US may have special rules(easier?) for export to Canada compared to some other countries. You will have to check with them though.
Regards,
Anand