I've been using Zencart with a Canadapost module for over a year.

I want to migrate the store to Drupal. Dimensions are essential to accurate price generation with a Canadapost shipping module for a retail store such as the one I'm working on.

Can't wait to see that happen! If I can help, I'll gladly join in the effort.

From my impression (I have not used another shipping service than Canadapost) the important difference is that you can specify boxes of any sizes to the canada post server - they don't provide standard box sizes.

The Canada post server receives product dimensions and fit them into whatever user defined boxes are recorded in the retailer's canada post profile - unless the item is marked as "ready to ship", in which case the canada post servers provides a quote using the product's dimensions as the shipping box.