How to dispute a roaming charge?

Oof, that might be hard to dispute, if you used the data while abroad. You could schedule a callback and see if they will show some leniency, but it really depend on the status of your account and the goodwill of the rep. Wishing you good luck though :)

$40 sounds like more than 1 day of easy roaming charges.  How many days were you charged for easy roam? 

What triggered the easy roam charges?

You can Follow the steps in the article

It’s 2 days of easy roam -- when I arrived I quickly turned my phone off, didn’t use it for anything (though not quickly enough!) and then right before I left I turned it on to receive a 2FA incoming text message. Each day was $18. The “ER/VC” charge on my phone bill seems to be the only thing during the dates of the trip, so it was just roaming not being turned off. 

The ER/VC was there to mark events triggered easyroam. You can check data/SMS usage at that moment to see how Koodo calculated roaming in your case. If you found it incorrect, you can always dispute directly with Koodo 

Did you follow the all the steps in the article?

Was there any indication of sms sent, talk used around the time ER/VC was triggered (you will need to consider the timezone of course)? 

If not then it sounds like data was used. Which would mean you would see in your usage page in self serve some data was used. If this was the case, how much data was used?