No other country of origin, such as imported from the UK going through the USA into Canada or Mexico, is included under the NAFTA trade agreement. It applies only to products 'wholly' derived & manufactured in North America.
There is a 'process' that allows goods imported 'temporarily into the USA and then exported to say Canada to apply for a refund of duty. But it is not an an easy process or by any means guaranteed.
On the subject of Fedex and UPS - yes of course they charge a fee for customs clearance. It is generally around $25 per shipment which I can tell you is M-U-C-H less than other commercial shipping operations. This is because Fedex and UPS have set up their own operations to clear goods instead of 'hiring' a broker.