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

If you feel that you are being charged too much, or if you can't find the detailed invoice, 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 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 receive a reply if you contact them via social media.