If you do not have any unpaid invoice under your Servage account then you can not enable auto-renew option now. But you can do once you get one unpaid Invoice.
Please follow these steps:-
-Open the unpaid invoice in your control panel, if you don't have any open invoice then this process can't be done now.
-Click on 'Pay' tab at bottom
-Now for Payment method: Choose Credit card
-Enter card details. Select 'Save credit card information ' tick box
-scroll down, Now select 'Pay invoices on expiration day automatically' tick box.
-Then go in your control panel to My Account > Account > Account settings > Payment options and check for your default enabled auto charge payment card.
That will enable auto payments from next invoice onwards.