Great question! And there is no single answer to this.
For items from USA and UK:
You should look carefully at the listing in list view, not gallery view, on a desktop computer. If you see, beneath the price, the comment "customs service and international tracking provided", then the seller is using a program called the "Global Shipping Program" or GSP. This means that you will be charged applicable taxes and duties, shipping charges and a fee for preparing customs paperwork and clearance., by Pitney Bowes - this will show as a separate charge on your PayPal account. These listings will also take more time to be delivered. Large and/or heavy items, and very expensive items, may have a lower total cost with the GSP. However, there have been many reports about damaged goods. Most listings that offer the GSP are not good for Canadians, and there are many discussions on these boards about that - you should read them.
Usually, smaller purchases, under $100-200 Cdn and sent by US Postal Service(USPS) or other countries' postal services, come in to Canada without assessment of any extra charges. It costs more for CBS to examine and assess these things than what they collect. If these charges are assessed, you will receive a notice from Canada Post to pick up your parcel, and in addition to the tax and (applicable) duty charges, Canada Post will charge you $9.95 for collecting the fees. Those sent by couriers like UPS, FEDEX, DHL etc are always assessed and you either need to pay at the door upon delivery, get billed later (Fedex), or you have to drive to the nearest depot to pay and pick up your parcel. I avoid listings using these couriers, or ask the seller before bidding or buying if they can ship using USPS.
The Free Trade Agreement applies only to items manufactured in USA Mexico and Chile, not just sold from there. Of course, the paperwork must indicate correctly the country of origin, something that American sellers are not especially good at doing.
I hope this helps.
