I don't see how you can lock a firearm in a cardboard box. Of course it is up to you, but if something happens like the firearm comes out of the box in transit or you have to make an insurance claim I can't see that going well if you don't follow the rules. I have always used well wrapped cardboard boxes in the past with no issues but will use a hard case with a padlock in the future. This will doubtless increase the cost to the buyer.
https://www.canadapost.ca/tools/pg/manual/PGnonmail-e.asp#1389620
unload the firearms - there cannot be any ammunition in the firearm or in the package (bullets, cartridges and other ammunition are dangerous goods)
attach a secure locking device to the firearms
lock the firearms in a sturdy, non-transparent container, and
remove the bolt or bolt carrier from any firearms (if removable).
Firearms cannot be shipped via air and cannot have any markings on the outside of the packaging. The customer is solely responsible for meeting all Canadian Firearms Centre regulations.
https://www.canadapost.ca/tools/pg/manual/PGnonmail-e.asp#1389620
unload the firearms - there cannot be any ammunition in the firearm or in the package (bullets, cartridges and other ammunition are dangerous goods)
attach a secure locking device to the firearms
lock the firearms in a sturdy, non-transparent container, and
remove the bolt or bolt carrier from any firearms (if removable).
Firearms cannot be shipped via air and cannot have any markings on the outside of the packaging. The customer is solely responsible for meeting all Canadian Firearms Centre regulations.