Dan, did you get this resolved?
I've installed the Canada Post shipping module on OSCommerce Bootstrap Gold.
I am getting the same error message and additionally, checking the Disallowed Shipping Methods doesn't stick either. For example, when I edit the module's options, click on some of the methods (mostly the world wide shipping methods), click save, they don't seem to stay.
I am not getting the "unknown error from Canada Post" message, just seeing "" Shipping is currently not available for the selected shipping address. Please select or create a new shipping address to use with your purchase." "