You can find a detailed specification of the amount you are being charged for in Mijn NS (go to Betaaloverzicht/My payments). Perhaps that will clarify things.

If you feel your invoice is incorrect, then you should contact customer service directly. Usually, this is most easily done by phone, as they are available 24/7 at 030-751 51 55. You may also try to contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook. Note that it can take a few days to get a reply via social media.