You're constrained by a mixture of US and Canadian laws. Broadly, the US ATF forbids any individual from hand-carry importing in excess of the $100 USD ITAR exemption. Canada further restricts what you can personally import (no receivers - upper or lower, bolts, mags or barrels).
Practically, this means you can carry across cheap accessories and parts such as stocks, grips, rails, LPKs, etc., as long as the cumulative purchase price of all ITAR controlled items in your possession is below $100. Canada customs doesn't actually care how much you bring across, as long as it isn't one of the import-prohibited items, and that you declare it for tax payment. This being said, the ATF doesn't mess around, and I've heard anecdotally of check stops near border towns on busy shopping days looking for ITAR items in excess of the exemption limit. Not worth chancing.
Echoing comments above, you're typically better off just buying through Brownells, or using an importer like Prophet River if you have a single expensive accessory (complete upper receiver, high end optic, etc.).