When shipping to Canada, I think NAFTA only applies for items assembled and manufactured in the USA. If it is manufactured in the USA and assembled in China, for example, there is some partial tax for importing it into Canada. The import tax book covers this, and numerous other similar conditions. It may also cover the origin of the raw materials used for manufacturing. From personal experience, I have determined that Canadian customs completely disregards NAFTA and taxes you on whatever they feel like it.
However, you usually don't get taxed on USPS First-Class Mail packages with a claimed value of less than $100. Apparently, it isn't worth the effort. However, for packages valued at over $100, Canada Post has been contracted by Canada Customs to collect a brokerage fee now. I think I had to pay $38 in taxes/tariffs/brokerage for a $135 item.
Also, from what I recall, packages declared as gifts coming into Canada are only tax-free up to about $70. I personally think that limit is way too low; think about wedding gifts from relatives, birthday gifts for children, etc. Generally, gift packages pass through Canada Customs much faster than non-gift packages.
In reply to the original poster, I've determined that the eBay shipping calculator is not necessarily reflective of all shipping options, nor of the actual shipping costs. I do find that some sellers are still knowingly taking advantage of this, while others have no idea it was so high. Most of the time, I furnish some evidence of what the actual USPS shipping fees will cost and up it by a buck or two for packaging. For sellers who do not ship internationally, and who have not blocked international buyer messaging, I try to offer some incentives to sell to me, such as,
1) Willingness to pay them with my Paypal account balance instead of a Paypal credit card. This saves them credit card fees.
2) Willingness to accept the package as-is for shipping via USPS First-Class Mail. Lost packages, no problem, just maintain proof that you actually shipped the item to me. I have never lost a package, btw.
3) If they reply to the likings of, "USPS First-Class Mail has no tracking!" I like to mention that I have received hundreds of packages from the US to Canada and have never lost one. Shipping to countries other than Canada may have a higher package loss rate, but not to Canada, there is no sea between us!
Items 1-3 are usually successful, but some sellers can be either overly stubborn, or overly cautious. For those rare cases, it is always helpful to have a friend in the US with an eBay account. I usually pick-up the packages on an eventual trip down south. If you are absent from Canada for 1 week, you can bring back $750 in goods tax-free.
Plan your life wisely, you'll be dead before you know it.