I did customs paperwork for a logistics company for a lot of years for products shipped from business to both business/private citizens back and forth (US to Canada and Canada to the US).
The ease at which it passes through customs really depends on how well the forms were filled out by the shipper. If buying from a business they are usually well used to adding TSUS numbers (just a number that identifies what the item or part is and what duties/taxes apply to the item on whichever side of the border) - having the number on the documents also saves customs trying to guess at what the description means and possibly opening the package for additional screening.
Now, it's been a few years, but items that were made in Canada or made anywhere in US/Canada/Mexico that were going to the US were duty free "period" - your State may however, collect sales taxes.
In general, for items made outside of a NAFTA country, the duty exemption was $250 for items bound for the US and a "whopping" $20 for items coming into Canada.
US Customs does want to know: where it came from, what is it, what's it worth, who it's going to and is it dangerous. Canada Customs has a single question: Who is going to pay the taxes
And yes, the post office's on both sides of the border have reciprocal agreements as far as tracking etc goes and they handle the "brokerage" for minimal bucks. UPS will burn you on "brokerage fees" - if you use a courier, FEDEX is far more economical on the "import charges". If the Canadian Company ships "Purolator" you will most likely get your package from USPS (whatever your express parcel service is from them).
If I was "personally" shipping an item to the US, I would use FEDEX (which I do for my Flea-Bay shipments - cheaper than Canada Post and quicker service, but FEDEX is not a "common carrier" used by business up here).
And that's about as much as I remember..
