Welcome to the NS Community, Nhu Quynh Anh Nguyen.

The best possible explanation I have based on this information is that you already had NS Flex. Either another subscription or the Basic variant. 
The invoice date of your original NS Flex contract is likely in a few days. An invoice will always run from that date to the same date the next month. Even if you change your subscription in the middle of the month. This is most likely why you've received an invoice for only a portion of the subscription costs. On your actual invoice date you will receive an invoice for the full amount.

Of course I cannot say this is what's going on with absolute certainty, so my advice would be to contact Customer Service. They can take a look at the invoice and tell you exactly what is going on.

See your invoice, the € 2,63 is for 3 days, until your next fixed invoice date.

€ 34,95 / 30 (days in april/may period) × 3 = (round) € 3,50
With 25% discount (round) € 2,63

Your next (May) invoice will show € 34,95 and € 8,74 discount. The following (June) invoice will show € 34,95 and a smaller discount because the discount is only for a part of that month.