I think the only restrictions are for endangered species that are on the CITES list (Convention on International Trade in Endangered Species of Wild Flora and Fauna). Otherwise, I have bought seeds from US companies by mail order just like from any Canadian company.

If you want to bring in live plant material, then things get much more complicated, as you need a phytosanitary certificate and stuff like that, but that is not relevant if you are only buying seeds.
-Don

Other than having to sometimes pay duty on seeds received from the USA, I don't know of too many restrictions upon getting them sent over the border.

Terry

I order seeds all the time from the USA, the only place that I had to pay customs (gst tax) and their paper work for doing so was from Hardyplants.com.

You also need to know what noxious weed seeds are not allowed to enter Canada and or your province.

Good time to order seeds with our strong Canadian $ right now ;)

Sharon

I think for small orders that are usually just put in a small envelope, they are less likely to charge duty. Another place I have ordered seeds from (through ebay.ca) is the United Kingdom. Maybe it was due to the seller being extremely good at shipping items fast, but I received my seeds from them in about 5 days.

Glen