Unitedstatespostalservice

"Canada Post automatically applies a processing fee of $ 5.00 ($ 8.00 for Priority Post) for each package to clear
Canada Customs. "

Charges are determined by Canada Customs.
The products introduced in Canada are charged duties and taxes (GST / QST).
The duty is determined by the value of product and whether or not it meets the criteria of NAFTA.

Because of the requirement / handling / GST charges etc as this is not always a great saving to buy from USA and shipping to Canada. But sometimes it saves more money, or perhaps the only way to get the product.