Just to confirm, are you asking to cancel Amazon Music or cancel Amazon Prime?  

Koodo streamplus (which include Amazon prime in it) or Amazon Prime add-on could be managed in self-serve.

To add to Dinh, steps to either remove the Stream+ subscription or remove the Amazon Prime Add-on from your Koodo account can be found in the links below.

Just keep in mind that if it is Stream+ and you remove it, it will also stop the Netflix and Disney+ subscriptions and you will have to subscribe to them again through their respective websites. 

How to remove a subscription: https://www.koodomobile.com/en/help/how-do-i-cancel-my-koodo-internet-subscription

How to remove the Amazon Prime Add-on: https://www.koodomobile.com/en/help/changing-your-add-ons

Amazon music must be removed in your Amazon prime settings. It’s completely separate from your regular Amazon subscription. My child accidentally added it and it took me some time to figure out what was going on.  Frankly, I’m surprised it’s not working. What happens when you follow the steps here:

https://www.amazon.ca/gp/help/customer/display.html?nodeId=GCRZL3F2UZMNP3T3

If you wish to also cancel your amazon prime, then you can follow the steps given by Dinh/ Oliver.