WES does not charge by the number of degrees that you get assessed by them. So it would be advisable to send them all the degrees/diplomas that you have from Bachelors onwards (though the requirement is to get only your highest foreign degree assessed). So, if you have a Bachelors plus the diploma that you mentioned and WES assesses it as Bachelors plus diploma, you can select 'two or more degree where one is at least 3 years or more'. This will give you more points when compared to just getting your Bachelors or diploma assessed individually.
Meanwhile, if you would like to check the Canadian equivalency of your degree or diploma, you can check this link http://www.wes.org/ca/evaluations/preliminary.asp?
