Why is itunes charging me US$12.99 a month?

If you are being charged the same amount monthly then it signs like you could have signed up for a subscription, you can view and cancel your subscription following the steps at How to cancel a subscription from Apple – Apple Support

Please remember that free trials automatically continue as a paid subscription unless cancelled.

You could potentially try and claim a refund by contacting Apple's Form > Request a refund for apps or content that you bought from Apple – Apple Support however this isn't guaranteed and it is up to whether Apple allow the refund or not.