IIRC NS tried to charge 10 at the start and there was an outcry.
The supplement on Inter City Direct is only payable on the new high speed line, between Schipol and Rotterdam Central.
If you purchase a supplenet from a ticket machine with a Chip card, it will charge 2.30. If you go to the booking office and purchase it you will be charged a supplement for not using the ticket machine.
If you arrive on a train without a supplement ticket, you will be charged a 10 supplement, so beware.
If you simply want to try out Inter City Direct, without paying a supplement, you should travel from Rotterdam to Breda, where although the train still uses a part of the High Speed line to Brussels and Paris, no supplement is payable.