Question about American buying goods in Canada

Actually, it never occurred to me to declare the repair parts for the car. Had I thought of it, I would have.
When Customs asked if I had anything to declare, I handed them a printed sheet itemizing what we bought, how many, unit price and total price, with an overall total at the bottom.
IIRC, the law requires that you declare everything, even though there may or may not be duty required on it.

GM