Anything listed as having import duty will attract it when shipped. It will also attract GST.
When you clear customs in the airport, the customs officials go through the items and level duty there.
When you ship large amounts, they come through the port and customs officials clear and levy duty there.
When you ship through the post office, the postmaster is the legally appointed customs person - they are supposed to open the package and assess appropriate duty.
GST is also charged on the cost of the goods as well as the cost of shipping, so charging it on the price of the stamp from the originating country seems consistent with that regulation.
The postmaster levies and collects the duty, logs it in and should give proper documentation to the person receiving the package.
PS - a US postal worker isn't going to know any of this ......