If you wish to have your monthly invoices settled automatically you have two choices:
1. Create a Direct Debit.
On any invoice you will have a dropdown selection box under "Payment Method". Choose "Direct Debit" and on clicking the "Setup Mandate for Payment" button at the top of the invoice you will be taken to the Direct Debit Mandate screen to complete your details. All future invoices for that service will then be paid by Direct Debit automatically. A more detailed explanation is contained here: How do I setup a Direct Debit to pay my invoices?
2. Add funds directly to your account with us.
Under the "Billing" navigation menu item dropdown, select "Add Funds to My Account". Enter the amount you wish to credit into your account and process the payment.
Any invoices that are generated in the future will have the appropriate amount deducted from this credit balance. You can add funds either using a credit or debit card (processed via PayPal) or from your own PayPal account.
You may also wish to refer to the Knowledgebase Article What payment methods do you accept?