First make sure you are listing on eBaydotCA. It's in the URL line. (sorry to be so obvious).
Then in shipping Details first dropdown Choose Calculated Shipping
Then next dropdown Choose your service (It will not include Letter rates)
Then go to International Shipping
Choose Custom Location
Choose USA
Choose service
Choose Offer Additional Service
Choose Custom Location
Choose countries (but NOT USA or N&S America)
Choose service (Make it Air Mail , trust me on this)
Under Package Details enter dimensions and weight . Use METRIC (which is also in an options dropdown, fairly low.) Canada Post went metric in 1974. Everything else is inaccurate.