It's a good idea to set up autopay whether or not you intend on having the chosen card charged for renewal. As it's been covered already, you'll get a $2 per 30 days credit for having the service enabled. You can set it up and still add payment vouchers and still receive the autopay discount.
You just need to add your CC to your account under the payment tab, then select 'manage autopay' and follow the prompts to register your card and receive $2 discount every 30 days.
