Many US sellers will not ship internationally which is why they will not ship to Canada.

And you get Canadian sellers who cross the border into the US to get cheaper and faster postage.

When my in laws were alive we ended up posting gifts to my brother in law in Seattle knowing that he would be seeing his parents on the day.  This was after FIL gave us a bollocking for being late with MIL's birthday wishes....we sent it 6 weeks before the day as letter post, not even a parcel.