You should be able to find a detailed invoice in Mijn NS (the website, not the app). This should tell you what you are being charged for.

If you can’t find the detailed invoice, or if the invoice doesn't answer your questions or seems incorrect, then you should contact customer service directly for more information. This can be done by phone, as they're available 24/7 at +31 30 751 51 55. But given that you're abroad, the easiest ways to do this are probably chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook.

I received an invoice and i read i need to pay till today or before today otherwise my bill will be send it tomorrow to another agency. I have Just paid by transfer. What shoud i do to avoid this extra cost?

Hi Vane1304! If the letter says that you had to pay before today, then the invoice will have been transferred to the collection agency today. If it said that you had until and including today to pay, then this transfer did not take place. It takes at least 24 hours for a payment to be visible in the system. I advise you to contact the Customer Service tomorrow to find out if your payment was received on time.