I am having the same problem, and I am also as equally confused.
it seems if you set the wieght of your item to zero the shipping wieght through canada post start at 3kgs, but no matter how musc you order the wieght always stays at 3kgs ( because you started out by setting it to zero )
it is a tough one, I am looking for some answers my self if I find anything I will be sure to post it