To save your billing information, please just follow the below steps:
During the checkout process, after entering your payment information for an order, you will see a check-box. Click the check-box to save your billing information.
Note: Currently if you have saved your billing information, you can update it, but not delete it, We hope to have that option available soon, but in the meantime, please contact Customer Support (links below) and we will be happy to remove it for you.