I get an error invalid shipping method when I select a shipping method from the Canada Post extension.
Possibilities 1. If you are using a custom theme, please make sure you copy all the files from the archive in their proper locations. If you have a custom theme, the files under app/design/frontend/default/default/ need to be copied under app/design/frontend/<custom_package>/<custom_theme>/.
2. Ensure products do not have "0.000" weight. Canada Post requires that all items have weight. Without a valid weight value, you will not be able to receive rates on your site.
3. Ensure logging is enabled in System > Configuration > Developer > Log Settings. Also ensure that Canda Post 2.0 API is in debug mode, System > Configuration > Shipping Methods > Canda Post v2 > debug mode. Recreate the "Invalid Shipping Method" error, and look in the log file to find out why this error is occurring.