Canada Post will be cheapest.
There are three basic services from Canada Post.
If the item is under 500 grams and less than 2cm thick, an International LetterPost item will cost you $14.65. The almost identical LightPacket will cost you $16.60.
LP is Air Mail but it is not insured* nor is it Delivery Confirmed. Only use it if you can afford to refund your buyer.
Small Packet Air allows larger parcels. It is insured against loss or damage in transit. It is reasonably fast. It is not Delivery Confirmed. If there is a dispute, your loss is covered by Canada Post, although the process is slow.
Do NOT ship Small Packet Surface. It is slow and you will refund for non-delivery. In fact, USPS gave up on surface international shipment a few years ago.
ExpressPostInternational is insured, very fast and usually Delivery Confirmed. It is expensive.
You can see more details about these at http://www.canadapost.ca/tools/pg/prices/Default-e.asp
It's complex, so print out thePDF files and plan to think about them.
*There are third party insurers who will cover shipments not insured by Canada Post. One often spoken of here is shipinsure.
