Looks like you already had NS Flex with an original starting date and invoice date of the 17th. The invoice date remains the same, even if you change to another Flex subscription on another date of the month. So after the initial invoice for part of a month (March 28 until April 16) for you a new invoice month starts on April 17 for a full month. However, you can change it back to Flex Basis on April 28 if you only want it for one month and you will get a refund from then until May 16. 

With NS Flex your invoicing date is fixed at the date you've first subscribed to Flex, in your case the 16th of the month.

As your invoice states you paid less for the first period of WeekendVrij as it was not a full month from the moment of switching to WeekendVrij till your invoice date. From the 17th you got a full months fare invoices, but that doesn't change when you could switch back to basic or any other subscription. That's still possible at the 28th and you'll get back what you have paid to much should you switch to a cheaper subscription or just pay the additional amount when switching to a more expensive subscription.

Welcome to the NS Community!

Subscriptions are always billed (monthly) in advance but eventually, you only pay for what you use.

So if you switch back to Flex Basic or Dal Voordeel on April 28, you will get 19/30 days back from the € 34,95 you just paid. In that case, you will get € 22,14 back.

See your invoice for 28-03 to 16-04. You then got 20/31 unused days back (28-03 to 16-04) from the € 5,60 you paid for the Dal Voordeel subscription you had. (20/31 × 5,60 = € 3,61)

If you switch back to Dal Voordeel on April 28, you will pay € 3,55 for 28-04 to 16-05 and then € 5,60 for 17-05 to 16-06 and so on… 

Thank you everyone, for the clarification.

You’re welcome, glad to help.

I do see an error in the 25% discount for Weekend Vrij in your screenshot.

For 28-03 to 16-04 you are billed 20/31 × 34,95 = € 22,55 which is correct.

25% off would give € 5,64 discount/korting, not € 3,64

The settlements on the invoice clearly show that sudipkundu had an NS Flex Dal Voordeel subscription before. If that was for 2.50 euros per month, then sudipkundu already received a discount of 20/31 × (5.60 − 2.50) = 2.00 euros.

Dammit… the cat nailed it 

Stupid discounts… so @sudipkundu got € 3,61 back (20/31 × € 5,60) which should be € 1,61 (20/31 × € 2,50) 

So far for the ‘missing’ € 2,00
Not an error but still very confusing/unclear