That could be due to a number of reasons. Did you order NS Flex Weekend Vrij for an existing OV-chipcard and did you not have NS Flex before? Then you need to load the product on your card at a ticket machine. Did you travel by bus, tram or metro? Weekend Vrij only allows for free travel by train.

In any case, 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. 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 online.