You can add a credit card on your manage payment page. If desired, you then can set this saved credit card to be used as your default payment method. Adding a saved credit card also makes making a one time invoice payment quick for you can choose to use a saved credit card.
You also have the option of adding a credit card while making an invoice payment.
If this is the only saved payment method, it will automatically be set to default; if you add additional payment methods, you can select a new default.
You can view and edit your saved payment methods on your payment management page