Hi,
I ran a quick test by selecting the ebay.ca site (page top). I selected "Calculated" and all the Canada Post methods were in the drop-down list. I selected "Canada Post Expedited Parcel" and the gray dimension boxes turn white for entry. In checking, any of the 5 Canada Post methods selected would allow entry of dimensions.
From past experience, I do believe at least one qualifying service-method must be selected with calculated.....that combo drives the calculated use for listing and checkout.
Hope that helps.
Danno