Duty is charged on item imported into the country at various rates ... there is a huge CRA document that lists all the various rules. Usually, stuff made in USA or Mexico are duty free (under NAFTA). Most other stuff is not duy-free, but it is rarely charged ... for some reason, they customs just assumes it's made in the USA if it comes from there. The duty on German stuff (like BMW parts) is 6.1%.
Now, on top of that, there is GST and PST which is charged on all things, just like buying at a store. Some things are exept again (like wheelchairs) but most items get charged 13% in addition to the duty.
So, on a $100 item (CAN value) if it's not made in North America, you'll usually pay 6% (duty) + 13% (GST + PST) = $19. Sometimes, the duty (6%) is not charged ... no idea why. Now carriers, like UPS will hose you for an additional "brokerage fee", which can be huge (like the $105 in your case, or $50+ from UPS).
2007 BMW 328Xi Touring, 6 speed