Duty fees US to Canada for auto parts?

I am looking at purchasing a part from Rockauto's website, and was wondering if I would need to pay duty fees on this order. I live in Ontario. The price with shipping and taxes is about half of what they want in a parts store here in town, so it seems appealing but wife warned me about duty fees...I thought that was the point of this NAFTA, where there was no duty. Has anybody bought auto parts online from US that lives in Canada?

I have made many purchases from Rockauto. Living in B.C., I only had to pay GST. My last order only had $1.46 duty charges. I usually try to order many items so shipping costs can be divided between all the parts. Chuck's