Did you cancel or change your subscription? Because that would be a reason for NS to refund an invoice. Simply not using a subscription is not a reason to refund it. If you don't use a subscription you can cancel it or change it. If you have NS Flex with a subscription you can change it to NS Flex without subscription and then you won't get an invoice for a subscription. When you change or cancel a NS Flex subscription halfway a month you will get a refund for the remaining days already paid in front. But you yourself need to take action to change or cancel it. But as long as you don't take that action and just not use a subscription there will be no refund.

In any case, you should contact customer service directly for information about refunds. This can be done by phone 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook.