First, log into your Roku user account (https://my.roku.com). You can see any subscriptions you have with Roku and can make any changes there. If nothing shows there, then look at the Purchase History to see if anything shows there.
If you find nothing in either place, then Roku is not the one billing you. It doesn't matter if your CC statement says Roku, as scammers can put anything they want in that text field and they often use Roku hoping people think it's a valid charge and just pay it. Again, if there's nothing showing, you are being scammed. Immediately contact your CC issuer, contest any charges you feel are bogus, and ask them to cancel that card number and issue you a new card. If you are being scammed, then your CC information is "out there" and others may attempt to do the same thing.
Dan
Roku Community Streaming Expert
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