There is lots of information about cross border ordering on this site.
Firearms, their parts and optical accessories are export controlled by either the US Departments of State or Commerce. Commerce is responsible for sporting optics and accessories and sporting shotguns, State for most other goods.
There are some exemptions and exceptions from the requirements for US export permits. Some items only require a SED rather than an export permit.
Cabelas, as a matter of company policy will not ship any firearm related goods to Canada, regardless of licencing requirements. They have obviously made a decision that they do not want the business.
If a package containing controlled items is detected, it isn't going to be returned to the sender. It is going to be confiscated, and there is every likelihood that the seller is going to be charged.
One large mailorder business was fined close to half a million dollars for the unlicenced export of 'scopes.
There are some US businesses that are either ignorant of their obligations under US law, or are knowingly ignoring the law in order to make a few extra sales. There are others that accept export business, and comply with law, even if it does make the sales much more complicated.
