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. Perhaps it was for a journey you made just before you left the country which didn't make it onto a previous invoice yet?

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.

If you are abroad, contacting customer service is most easily done by chat (click the link Chat with NS on this page) or by sending a message on X (formerly Twitter) or Facebook.