Do you have access to volume discounts that require a minimum monthly shipment count? I'm asking because NetParcel in conjunction with PayPal offers the lowest rates you can get without committing to a certain shipment count and I was wondering if your pricing would be cheaper than that. They don't offer Canada Post though so I guess you'd have an advantage there.

Pricing seems decent with Canada Post I suppose. For your Toronto-Seattle example, netParcel charges $23.82 CAD but it's 8 day transit with UPS. Canada Post at $35.70 for 4 day transit seems pretty decent and beats small business pricing as well as faster netParcel options.

edit: You should really negotiate your Australia shipping contract if you ship there occasionally as UPS can go as low as $114.57 CAD for 3 day delivery.