What does it say on the invoice? We can't see that here. The invoice should give details of what you are paying for.

Good morning Bassira and welcome to the NS Community.

If you changed your subscription to a more expensive subscription, you will have received an extra invoice shortly after the change to pay part of the subscription costs of the new subscription. Then, at the next monthly renewal date, you're paying the full subscription costs of that new subscription with the standard periodic NS Flex invoice.

As rvdborgt mentioned, you can find the full specification on the invoices themselves. You can find them in your Mijn NS account, under ‘My payments’.