You need an ATT to take it to the postal outlet unless your long term ATT covers you for that.
Shipping Guns by Canada Post
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Yes you can ship guns by Canada Post. See these links:
http://www.cfc-cafc.gc.ca/faq/default_e.asp#d3
Q. Can I send firearms in the mail?
You may ship restricted firearms, non-restricted firearms and prohibited handguns from one Canadian location to another Canadian location if you use the most secure method offered by Canada Post that requires a signature upon delivery. Prohibited firearms, other than prohibited handguns, and firearms being shipped across the Canadian border, must be shipped by an individual or carrier company licensed under the Firearms Act to transport those classes of firearms.
You are required by law to ship firearms unloaded and in a safe and secure manner to deter loss, theft and accidents.
http://www.canadapost.ca/tools/pg/manual/b07-e.asp
Firearms (including imitation and replica firearms)
Please contact the Canadian Firearms Centre at 1 800 731-4000 to determine whether it is permissible to ship your firearms.
When it is determined permissible to ship firearms, they are to be shipped using Canada Post’s most secure product, Priority Courier with signature.
Remember to pack it well. DO NOT mark it as a firearm. Don't tell them what it is even if they ask. There is no requirement to do so and postal theft is a big problem.