Hi Derar! Welcome to the NS Community!
Did you download and view your invoice to find out what this amount is for? If not, then I recommend to do so. Here we explain how to find and understand your invoice. If you still have questions about this invoice after you've downloaded and viewed your invoice, then you can contact the Customer Service.
The invoice amount is minus €66,43, NS will pay you this, you don't have to pays this to NS. When you get NS Flex the money that was on your OV-chipcard is removed and payed back to you on the next invoice after activating the subscription at a machine.
The invoice itself will explain what the (credit/to be received) amount is for.
It could be balance/saldo that was removed from your card or restitution from a former non-Flex subscription (e.g. Dal Voordeel op saldo that was recently renewed and costs € 71,40 per year).