A little more info.
After 1/2 hour on the phone with CP, I have been able to determine (you probably already know this, but it may be of value to other newbies like myself) that the shipping module should not be used for "Xpresspost Prepaid Bubble Envelopes" - the key issue here being "prepaid".
Since the module communicates with sellonline it evaluates an item based on dimensions and weight.
I will therefore have to setup non-prepaid bubble envelopes and enter them into our Retailer Profile at CP like they were boxes. For example, a standard bubble envelope is 24.5 cm long x 21.3cm wide x 2cm deep/high and its maximum weight for envelope and items being mailed is 500g.
Canada Post tells me that the shipping cost will be evaluated as "parcel post" and therefore will be cheaper than the prepaid bubble envelopes!