When I lived in Canada I would just file GST taxes everything as 0.
I would not charge GST because obviously its impossible to do. I would get revenue Canada phoning every couple of months or get letters in the mail and each time I would remind them that I do not deal with Canadian businesses (GST is only charged to Canadians) and that all my clients were international. That would get them off my back.
I would not claim gst back on my purchases. I tried that once and they never sent the the money they owed me so it was honestly not worth the effort, just easier to write a bunch of zeros.
The crap thing is once you earn over 30k a year you have to file it 4 times a year.