You can find a detailed invoice in Mijn NS (the website, not the app; go to Betaaloverzicht/My payments) to see what you are being charged for.

If after looking at the invoice you feel that you are being charged incorrectly, or if you can't find the detailed invoice, then you should contact customer service directly. They can tell you what the invoice is for, and correct an incorrect invoice.

Usually, contacting customer service this is most easily done by phone, as they are available 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.

You've probably got a subscription with NS that either gives you a right to a discount, which you need to pay for unrelated to how much you acgually use it, or you've used after the last trip that was included in your last invoice. Check your invoice, it specifies exactly what you're charged with and whilst in the second case it is probably okay and you won't be charged again, in the first case you need to take action to avoid being charged again next month.