Just get a Canadian P.A.L. (and register your shotgun under their long gun registry) if you're going up there on a regular basis. It makes customs a snap. With the PAL you can buy ammo or even a gun with no problem. You can also carry 200 rounds into Canada in baggage, having it shipped may be different. I just buy it up there. To bring a gun you purchase in Canada back to the US is more of a problem...Geo
I live in Canada where quite a bit of waterfowl hunting takes place. My brother lives in Minnesota and he got his PAL and registered his guns in Canada. As mentioned, it makes it easy to bring them in, there is no wait at the border and the licence is good for 5 years. It also says in the Canadian firearms laws that an American can sell a gun to a Canadian (when in Canada) if it has been permanently registered in Canada, meaning you have a PAL. This may be an option for people trying to sell a gun to someone in Canada. Once you are in Canada, it becomes Canadian Law. So if a fellow up here on a hunting trip decides to sell his gun to someone, according to Canadian law it is legal if the person has a Pal. You do not have to go through a dealer here to sell privately. You phone down to the center and transfer over the gun, no cost.
