Tim,
Not that assuming is always good, but I would think that if you purchasing using a credit card, then your credit card would be on file. That should do it. After, though, you may wish to send an email stating you would like to be on autorenew and send it to help@amazingcharts.com. Ask for a response. If no answer from them send back with its being copied to jon@amazingcharts.com.
Also, are you using this phone number: (866) 382-5932?
