I get that, and, yes, you should take the fee and GST into account when deciding whether or not to place the order. However, it seems obvious that Canada Post is pretty random in how they decide when to charge the handling fee and collect GST. I'd say the odds are higher that they let things slide, at least for duty free items, rather than do things "right", so it is a bit of a surprise (but not one you can complain about) when they do collect.
My experience bringing things back across the border is the same. On Saturday I drove down to Blaine to pick up a new pair of Mavic shoes that I had bought from REI and had delivered to a parcel receiving place. I was in the US for less than 15 minutes, declared the full value of the shoes coming back into Canada, and they waved me through. I've done the same dozens of times for non-bike stuff (when the dollar was at par, and my girls were small, we used to bring back, literally, trunk loads of diapers from diapers.com). I think we paid taxes maybe 10% of the time.
And if they charge the fee, it is just the taxes. Customs is obligated to clear consumer purchases, unlike business transactions which they make you do the paperwork for.
And most american made bike products only get the GST charged. :)