It depends on how the package is manifested when declared by the courier company to customs. This information is taken from the packing slip completed by the shipper (read volume retailer in this instance).
If as is sometimes the case, the goods are declared as 'samples, no commercial value' then this is clearly a misrepresentation and is tax avoidance from the perspective of local Customs and Excise at destination.
Still with me? It's boring, I know
Sometimes, the value of goods can be under-declared since there is a threshold where it becomes more expensive to collect the duties and taxes than it does to just let the goods in for free. It's likely that if you bought, for example, a pair of tyres that you would be under the threshold and not pay d&t. On a gruppo, I would suspect that d&t would be payable in Canada and the US.
If you flip this around, bringing these parts in to the UK would incur 15% import duty on the manifested price and 20% VAT payable on the manifested price plus duty. The manifest threshold in the UK is about $115 iirc.
I think that this is the illegal behaviour being referred to.