If you book your journey via NSInternational.nl or NS International Customer service, you will automatically receive a booking confirmation by email. The confirmation email doubles as an invoice and shows the amount of Value Added Tax (in Dutch: BTW) that applies to the Dutch part of your journey. You can find it underneath the total price of your booking.
If you book at a NS International Service desk, upon request you can received a printed copy of the booking confirmation or have the booking confirmation sent to your email address.