Sell Online is something different that the Canada Post Module for Zen Cart. You can use the former to make a little online shop on Canada Post server, or to generate shipping stamps and such but it does not integrate in any way with Zen Cart.
When setting up Zen Module you have to get a free account from Canada Post - that will give you an id name and password, both probably starting with CPC_
You then access that account from the Canada Post Module side bar in admin/configures/modules/shipping/CanadaPost
That is where you define which services you will use and define the box sizes you will use.
For each product, you have to give a length, width, height in your product description. In the Canada Post module you have to define boxes that will hold these items. AND you have to give the dimensions in the same sequence...ie don't use width, height, length, instead of length-width-height.
If you want to combine items for shipping you need to define additional boxes that will hold the multiples. And you need to keep the lengths, widths, and heights properly sorted - the module won't figure out how to turn one item sideways to fit in a box. You don't have to do this, but if you don't it will calculate shipping for three separate boxes for three items instead of one bigger box.
And likewise, you want a big enough selection of boxes in your defines that it doesn't put a 6" item in a 36" box.
I've been using CanadaPost module from zc 1.3.7 all the way up to my current 1.3.9h (I've been too chicken to upgrade to 1.5 yet - waiting from someone more knowledgeable than I to update the CP mod.)