You will not be charged for the journeys in a particular month, you monthly payment will be calculated based on the day that may vary and can be on every day of the month. Mine is calculated e.g. on a base of the 24th of the month. It is based upon the date of your first NS Flex subscription.

You can find a detailed specification of the costs in Mijn NS (the website, not the app; go to Betaaloverzicht/My payments). That will show you exactly why you have been charged this amount.

You could also contact customer service directly; they'll be able to tell you what you are being charged for, and correct the invoice if you are being charged too much. This can be done by phone; 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.