Hi .. Depends on value (excluding shipping paid to seller). Over $20 CDN , Customs can apply duty and taxes plus , by mail , Canada Post charges $9.95 for collecting. Many low cost items slip through without but can't count on it.
Most US sellers shipping courier , UPS/Fedex, ship ground service and Customs charges will be applied along with an expensive brokerage based on declared value. To be avoided. Ask seller if they would use mail.
Some US seller are using ebay's Global Shipping Program with import charges paid when checking out. Will see on listings.