Extremely accurate USPS international postage rates are available online here at the USPS website:
For example, a box weighing 1/2 pound costs $3.05 to mail to Canada by First Class mail.
A full pound goes to Canada for $5.01.
Delivery takes about a week or so.
In my experience, this is a very reliable service and the one I always chose. I also send by the equivalent low cost Air Mail service when selling, unless the buyer requests and prefers to pay more for another service.
Lately, many eBay sellers in the USA have stopped selling outside the USA. Many who do sell insist on arbitrarily high cost mailing charges at the buyers expense, even for low value lightweight items.
European sellers usually do not make this demand. They continue to sell around the world.
Emanuel Lowi
Montreal, Quebec