The form normally goes on the outside of the box. Many post offices seem to provide a little plastic sleeve to put it in - the open end self-seals or gets taped closed. Usually, Canada Customs just read the form without opening the box. These days, they may run it through an x-ray machine to make sure there isn`t a gun inside. We are not big on letting guns into the country - or fresh fruits and veggies, or living plants. Otherwise, the border is pretty open - just think of all those white boxes which go across to the Point Roberts Post Office!
Salt Spring Island