Duty and sales taxes are due on any imported item valued over $20.
CBSA and Canada Post were instructed to ignore these if the item was under $100. Which is illegal , but hey, this is Harper.
It would have made more sense to change the law to allow imports up to $200 as is the case when you drive across the border. So not only scofflaws but lazy scofflaws.
Commercial carriers are required to charge duty and tax or be prosecuted.
Even if there is no duty, there may be sales tax, which can add from 5% to 15% to the cost of the item. There will also be a small fee basically for brokerage from the processor.