PSN has automatically renewed despite insufficient credit?

My PSN subscription was recently renewed automatically even though I hardly had any credit on it (I also didn't add a payment method). Now the question arises, how do you charge me for that? Does anyone know?

Subscriptions with a term are always extended automatically (usually by 1 year) if nothing else is in the contract.

They send an invoice in the mail. If payment is not made, there are reminders and then a judicial order for payment etc.