The widget that shows the subscription costs in Mijn NS is notoriously broken. Especially after a subscription change it shows wrong information. Ignore it, the invoice will show the correct information, i.e. no subscription costs.

Consumer card number *card number deleted by moderator*

The price of 37.70 seems to be weekend free with 1e class supplement.

Have you buyed the subscription with 1st class?  

If you changed Weekend Vrij to Basic and received a confirmation e-mail about that, you should be good.

As @Tochjo stated, the cost counter in Mijn NS (website and app) is notoriously off. You will not be invoiced for that.

If you wanted to cancel NS Flex altogether (go back to travel on balance), you would have to go to a machine (station, supermarket etc.) to completely remove it from your card.

Within one month, or else your subscription will continue.