I am from Canada and one of my sources is from the states, I have ordered from him numerous times and have never lost anything to customs.
So, to answer your question "Is it just as difficult getting gear into the US from Canada as it is from an overseas source? Are incoming packs from Canada looked over as closely as packs from a country such as England or Romania?"
It is much easier to get gear from the US to Canada or vice-versa than into either country from overseas.
But, as Namu pointed out, there is always a chance it could happen.