It can be a little complicated. The importer/purchaser in Canada will be responsible for paying the federal Goods and Services Tax (7%), Provincial Sales Tax (anywhere from 0 in Alberta to 11% in Newfoundland), as well as customs duties (typically around 3% to 7%) on goods manufactured outside the USA or Canada. The tax and duties are calculated on the declared value of the item, but be careful about mis-declaring or under-valuing the goods, the importer could get slapped with a penalty. I recently bought some van den Hul cables from a UK-based on-line retailer, for which duties were 4%.
I'm not sure about goods shipped to one or the other country for service, but returning the item to the country of origin for servcice, such as the Blue Circle example above, should not entail any taxes or duties on trip to Canada. Going back the USA, it would likely mean taxes and any duties on the value of the service or modification.
