You can find a detailed invoice for every deduction in Mijn NS (the website, not the app; go to Betaaloverzicht/My payments). That will tell you exactly what you have been charged for.

If after looking at the invoice you feel that you have been charged too much, or if you can't find the detailed invoice, then you should contact customer service directly. They can tell you what this deduction 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 X (formerly Twitter) or Facebook.