For economy USPS is the best route. If you want tracking, speed, or signature receipt, you have no choice but to go through UPS or FedX, and you will pay a MUCH higher price for it. I ship frequently to Canada....USPS tracking/insurance, etc does not work once the package leaves the United States.
Most often, when going with USPS I will ship via Priority Mail International, but make sure that the client fully understands that they take all responsibility for lost or stolen packages. Customs can be a real road block too....many times I have shipped a package, only to have it "held" in Customs for 3-4 weeks before they forward it to the addressee, with obvious signs that the package(s) have been opened and resealed by Customs.
Unless you use a worldwide service such as UPS or FedX, the package goes through different entities within each country, with different rules and procedures, none of which are compatible with the other....that's why tracking, insurance, and most other services offered by USPS are null and void once the package leaves the U.S.