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. On the first page, you see the total amounts, but if you look at the later pages of the pdf file you can see exactly for what journeys you have to pay what amount.

If after looking at the invoice you still feel that you are being charged too much, or if you can't find the detailed invoice, then you should contact customer service directly. They can tell you what the higher amount 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.