Are those commissions you're earning from clicks on your website taxable?

Probably not if the payer is not resident, and not a registrant, as in Amazon.com, Google.com

But... if you are advertising for a US corporation that is controlled in Canada, that might make a difference.

To be sure, you can ask them to provide you with confirmation of their status in writing.

Here's where you'd look for the rules!

GST/HST Memoranda Series 4.5.3 para 16-17 has zero rating provisions. 
If you meet those, you should obtain Appendix A letter proof of non residence in Canada in GST/HST Memoranda Series 4.5.1.

http://www.cra-arc.gc.ca/E/pub/gm/4-5-3/README.html

If you don’t meet criteria for zero rating then you look to P 18-24 and advertising specifically para 25-27 

If you don’t meet the zero rating provisions in para 16/17, it’s necessary to get Appendix B letter proof of non residence and non registrant status from the provider of the commission.

Appendix A & B letter are found in GST/HST Memoranda Series 4.5.1.

http://www.cra-arc.gc.ca/E/pub/gm/4-5-1/README.html

I found out that less is more in the CRA search engine. If you type in GST/HST Memoranda Series and the number, you don't get anything, but if you just type in 4.5.3 you get what you want. Less is more; at least today...