It's a personal preference. I always chose to keep my self paid subscription separate from involvement with my required Roku account.
The only time I used a third party arraignment combination was through my former Walmart/Vudu combination account. Walmart could not help me when they withdrew from their Vudu's agreement, so it was up to Vudu's more knowledgeable technicians to separate my combo account from Walmart. Real messy.
So I really recommend keeping your accounts completely separate now. You (or even I) might not have such supportive over the phone guidance the next time!
We understand that you are still having an issue with being charged for a subscription, and it would be best to talk to the specified team since we need to gather your personal information. You can contact them here: support.roku.com/contactus then choose 'Questions about my account or billing' on the drop-down menu. They are the best ones to helpyou out with that matter. For further reference, you can also check Manage your subscriptions.