I only wanted to use the plan for one month and I want to deactivate it now. I can't see anywhere on the Self Serve page to do that. Is the only way to do that by calling Koodo and paying some ridiculously high service fee?
What do you mean by deactivate? Cancel? You need to call Koodo I'm afraid, but I'm not sure where you got the idea of a ridiculously high service fee? It's FREE to call Koodo unless you ask them to do something you can do yourself in self service :)
If you really mean to deactivate your plan, you can do that in self service, but you'd still be paying for your plan :)