If they want to bring it across the border, it's on them.
But SHIPPING gun parts (yes, stocks and forends are gun parts) is tricky at best when you consider that Border Patrol, US Customs, and US Dept of Commerce all have their own laws, as does Canada. You can also find yourself facing ITAR regulations, which is Dept of State, depending on what the part is and where it's going to. Shotgun wood has different regulations than rifle wood, and some rifle wood has different regulations than other rifle wood depending on the name and design.
For example, Boyds will ship shotgun wood internationally with no issue. They will only ship rifle wood up to $100 in value, except Canada, where the value for rifle wood has a $500 limit. Until recently, they couldn't ship their "Tacticool" design out of the country b/c of US laws regarding "tactical" gun parts. They changed its name (and NOTHING else, just the name) to "Pro Varmint", and voila, no issues and they can now ship internationally.
Best bet in the long run is to only sell gun parts to US residents. If the price is right and the product is good, you'll find a buyer eventually, and won't have any of the risks of dealing with international buyers.