pay invoice by balance

Hi Zohreh and welcome to the NS Community! 
Unfortunately, that is not possible. The invoices can only paid by your bank account.

You can go to an office at a (bigger)  station, so with people who work there. Normally you can become 30 euro’s at a time, but when you contact Customer Service before it may be a little higher. There are some formalities. 

I have almost 18 euros in my NS card! So, How can I use the balance on my NS card?

You cannot use it but you can get the balance back at a service desk.

See https://www.ov-chipkaart.nl/en/service-points-finder (choose Counter services).

Go to the office, get your money, put it on your bankaccount, pay your bill partly with this 18 euro. Your balance is not a legal mean to pay other than checking out or in.

Why did the device allow me to top up money on my NS card, and what are the advantages of having a balance on an NS card?

If you have an NS Flex subscription there are no advantages to having a money on the OV-chipkaart.
The top up system is not smart enough to be aware that there is an NS Flex subscription on the card before allowing you to top up.

Hi again
I paid invoice 440012532930 directly by bank card but didn't add any description about it!
How can you inform your company about it? (€19.30 to NS Groep NS Reizigers)

Good morning Zohreh. If you don't add a description the amount will ultimately be sent back to you. Please pay again with the correct description and the other payment will find its way back to you. 

Thanks, But in Revolut there is no box for writing the description. :(
I also prefer to pay the invoice again after the previous invoice has been refunded to me
How many days does the refund take?

Were you offered the ability to pay by QR-code or something similar by chance? There's no saying really how long the refund will take exactly. Usually this happens within 30 days.

No, it was not offered the ability to pay by QR-code to me.
So, I am going to wait for the refund, and then I will pay again. is there any deadline for the payment?   

Yes, there is. From the moment we've asked you to pay this you get 14 days of time to pay, with another reminder like it after the first. Waiting for this repayment might make it very close to the final deadline for this payment, so it's not recommended.

is there any email that I can send the transaction details to fix the problem?

There's not, no. The only way to fix this is as Sascha stated, to pay this again with the proper description. 

Hi 
I changed my account to NS-Basic which is free, and I did not use my cart after that , But I received an invoice today! why should I pay this amount?!

Look on the next pages of the invoice; you can see for what trip you're being charged there.