There are currently three billing methods available. For each of these methods you will receive invoices, the details of which can be configured from the Billing Preferences screen, by going to the Profile icon 
> Billing > Update Billing Preferences.
Monthly credit card payment
Your credit card account will automatically be debited on a monthly basis with the amount of your outstanding balance, which will usually be the amount of your most recent invoice.
Monthly Payment
You will be sent an invoice / invoices each month on your billing date (determined by the date on which you switched to a paid account) in respect of the following month’s usage.
Annual pre-payment @ 10% discount
You will be sent one invoice, which covers the 12 month period following your billing date. This method of payment attracts a 10% discount.
For the last two billing methods, payment can be made via PayPal.