Actually duty/tax can be charged on any package valued at over CA$20.
With free shipping the package is handled by Canada Post and CBSA. As a government agency they have the discretion to waive these charges. They often do that for under $100 packages, presumably because the amount of duty/tax is less than the cost of collecting it. But if they wanted to they could charge duty/tax on anything over CA$20.
With faster shipping options the package is cleared through customs by the shipping company, i.e. DHL, EMS, FedEx, etc. They are required by law to collect duty/tax on incoming valued at over CA$20. Moreover, as businesses who also charge hefty brokerage fees to perform this duty/tax collection "service", they have a strong financial incentive to do so.
Also most of the Chinese sellers like the two GBs and Aliexpress, usually undervalue the merchandise on customs declaration forms. At minimum they'll declare a US$50 item as $50 so that it appears to be CA$50. But more often they'll declare it at a much lower value. They also tend to falsely declare packages as gifts (where the duty/tax free limit is CA$60.) All of this is illegal but again Canada Post and/or CBSA generally don't bother to enforce. But they can and sometimes they do.