<<Disclaimer: Verify this information before applying it to your situation.>>

I just received an interesting response about this subject.  Here it is:

Just saw your post about customs fees. If the flour you purchased is made
in the USA, there should be no duty to pay. The manufacturer has to put a
commercial invoice in the shipment stating that it is made in the USA. If
they shipped it to you by a courier, there could be brokerage charges
involved to clear it into Canada. The easiest solution would be to get the
manufacturer to include a commercial invoice saying it is made in the USA
and send it by mail. At the most, you might have to pay a $5.00 handling
fee. Another way would be for them to send it by mail and put on the
customs declaration form that it is a sample and is made in the USA. I
didn't see your original post, so I hope this helps.

Ron (Vancouver, B.C.)