Summary:
If you make less than 30k/year in SALES with your self-employment or business, you don't need to register for taxes but you still can if you wish.
If you deal with Canadian companies as an affiliate, they will need to send you the tax if you are registered because you need to charge them as they are your client(like gamma for exemple).
When you purchase something for your business, you will pay tax, but at the end of the year, you can get it back from the govt, because you also charge tax when you sell your services.
If you sell to American companies, as an affiliate for example, you don't need to charge taxes(I clarified that with the guy from the Govt when I went at their offices to get my tax numbers).
Anyways, if you have any question, I would suggest you to consult your accountant and the Govt directly, as their authority on this is way better than some guy on a message board
