Ian,
In my personal experience import taxes in Canada are a "hit or miss" factor on hobby items, happily mostly "miss". The one most consistent extra charge I have experienced is when the shipping from the USA is done by a Courier Service in lieu of the USPS and an "Import Broker Fee" is sometimes incurred at the border. The biggest factor is the shipping (read postal ) fees which make many transactions prohibitive. That factor combined with currency exchange rates have driven me to shop mostly Canadian sources with the exception of the UK which still offers a reasonable deal on availability and price. The Canadian system is, at best, inconsistent. I once had a huge "Duty Fee" imposed on a Kit from Wingnut Wings in NZ when my wife picked up the parcel that all but doubled the price of the Kit, I would have refused it. That was the only one of more then a dozen purchased directly from the company that incurred an extra charge.
Those days of free shipping by WNW are a fond memory. Many orders are now priced below the cost of the postage to get them here.
Cheers,
Lance