Under the North American Free Trade Agreement, goods produced in the U.S. or Mexico are entitled to be imported into Canada free of duty.
For the commercial or non-commercial importation of a good whose value exceeds US$1,000 (or its equivalent amount in Mexican or Canadian currency), a Certificate of Origin, to be completed by the exporter, must accompany the goods. See below.
A Certificate of Origin is not required for the commercial or non-commercial importation of a good whose value does not exceed US$1,000. However, the invoice accompanying the importation must state that the good qualifies as an originating good, e.g., "I certify that the goods covered on this invoice qualify as Originating Goods under the NAFTA Rules of Origin."
You can access a 300 dpi bitmapped image that I scanned from Revenue Canada's form B 232E, in two formats:
Alternatively, you can access a much prettier one, in US Government dress, at UPS:
In Item 6 HS Tariff Classification Number, use:
In Item 7 Preference Criterion, use "A" for goods "wholly obtained or produced entirely" in the U.S.
Fill out the form, sign it, and send it with any commercial shipment that you are making to Canada! Presumably this form is also useful for goods shipped from Canada to the U.S.