If the taxes and processing fees aren't added to the item at the time of purchase, they should be added on at the other end around the time it's supposed to be delivered to you.
However, with many purchases from the U.S. sent by mail, you get "lucky" and Canada Border Services doesn't do the paperwork and the post office doesn't collect the taxes and fees due.
http://www.cbsa-asfc.gc.ca/import/postal-postale/duty-droits-eng.html
