Thanks for your inquiry about your billing charges.
For these issues, it may be more effective for you to get help from our billing team. Please contact them directly at https://support.roku.com/contactus. Choose the option from the drop-down menu for "Questions about my account or billing," and you'll be able to send an email request there.
Re: Subscriptions Are Being Charged after cancellation
If you subscribe to a channel directly with them, then you should login to them on your Roku and not set up Roku to bill you.
On the other hand, you can also subscribe via Roku and pay via your Roku account. If you do that, you'll manage the subscription and roku.com and the channel will generally not know anything about your personal account. (They just see some mass-subscription from roku.)
Either way works, but some people make an account at a channel provider and also set up the channel via Roku (ie: create a roku account for that channel rather than just logging in using established credentials with the channel.) If you do that, you will be billed twice because neither billing system is aware of the account at the other place.