You can add a debit or credit card to your billing account if you want invoices to be paid automatically. To do so, log into your billing account and go to Billing » Payment Methods. Alternatively, you can go straight to

An image of the Payment Methods page. The page lists any existing debit or credit cards on your account. There is also an option to add a new card.
The Payment Methods page. You can use this page to add a debit or credit card, and you can remove any existing cards.

To add a new card, simply click the Add New Credit Card button and enter your card details. If you are adding more than one card then you can optionally add a description (to remind you what the card should be used for).

An image of the form used to add a new debit or credit card.
Adding a new card.