Avoid them at all costs unless you can buy from a vendor that is using a version of UPS where duties, taxes, and fees are all pre-paid.
If you're not using UPS, who should you use? We recommend USPS, Royal Mail, or any other local post office. They don't charge any type of customs or brokerage fees.
The Canadian rule is that if the shipment is $20 CAD or less, it's not subject to taxes and duties. In some cases, you can ask a vendor to declare the item for less value than its worth.
There are of course insurance implications here so if an item is declared as less value, if it goes missing or is damaged, the coverage will be according to the declared value. This is why most companies won't do this. If it's friends or family, this shouldn't be an issue.
The other exception are gifts. Any gifts that are $60 CAD or less are also exempt from taxes and duties.