That's NAFTA in action, hi-ho. Duty-free transfer of instruments made in Canada, USA or Mexico (more of these are showing up, those most so far are low-end) between those three countries.

However nothing is ever as simple as that. For one thing if you are a Canadian resident and purchase a big-ticket item in the US and bring it into Canada, you're supposed to pay 7% GST on it.

Furthermore, there are potential complications with CITES regulations on importing instruments that have regulated materials like Brazilian rosewood or ivory. If the instrument can be shown definitely to be made before CITES came into effect (early 90's), there is no problem. If it's newer than that and has any regulated/prohibited materials, there is additional paperwork and you maybe can't get it...as usual, the 12th Fret people are the expert reference on this.

Does mid-Mo have an Octave mandolin, mooh?   

W-O