Hi Mathew,
let me pick up on what Parveen already said. I suppose you have not created a tax authority and company tax arrangement for your company in Canada. Whether you have/need a tax number in Canada I cannot tell, but the system cannot calculate and print taxes on the document as long as it does not know whether you are allowed to print taxes on the invoice.
So the system is correct in not putting Canada as the tax country. But each transactional document that can post taxes will determine "the best guess". So the system assumes that this is a non-taxable export from a US point of view...thus the determination of 0% tax rate. Consequently the tax return run for PA also collects this document...simply because every document will eventually be marked as "reported" no matter whether the taxes calculated in the document actually are required to be entered into a form or not...it is just necessary to complete the process.
If you need to report taxes in Canada though, you should maintain the tax authority and company tax arrangement in "Tax Management" workcenter.
Hope this helps.
Best regards,
Stephan