Welcome to our NS Community, Alessandratamb. I'm not sure where the € 0,90 came from. This is a public community, so I advice you to getin touch with our customer service for this. 

The transfer can be done from a different bank than is connected to your account. As long as the desciption is filed correctly, with either the feature number or your customer number and invoice number.

Check your account-that should still be available, on mijnNS. Can only be done via site., not via app on fone.

90ct is indeed a very rare/funny amount, as even a BUStrip will cost more as 1€. Maybe, perhaps, it could be a short bustrip after a change that was due to very slow working systems only passed on to NS/Flex after youre last payment. Or maybe putting a bike in some guarded bikepark.

As above: with IBAN/SEPA you can easily pay from any EU-bankaccount.

A few years ago I made a wrong check-in/check-out in the bus, used the wrong card. Because of that I had a check-in on my NS Flex card without a check-out, the cost for that was €0.90. Strange, because the basistarief for the bus should be €0.98. I had this corrected using “Uitcheck gemist”. My mistake happened on the 11th of the month while my invoice date was the 15th, the €0.90 cost plus the correction were both on the invoice of the month after that.

Perhaps this is also a situation like mine. I would check the travel history in your MijnNS account.