Welcome to the NS Community LPCBSC.

What a shame it went so wrong for you. 
It's currently very busy everywhere, so it can indeed take a little while before my colleagues can get back to you. May I ask for your patience? They will answer you as soon as possible.

E-mail is even slower… 

Thanks for the timeline 

The invoice is for last month. Do I pay it first or reject the invoice while I continue to seek a refund for the erroneous check in or out.  

The advice is to always pay the invoice. If it is too much to pay in one go you can ask for an arrangement. If my colleagues can correct the invoice for you, you'll receive a reimbursement on the next one.

thanks for the advice. How do I ask for an arrangement if the amount is too much to pay at one go?

I have got an invoice of amount 400 euros. I am using NS Flex Dal Vrij recently. I don’t understand why such huge bill are requested? can anyone explain?

You can take a look in your My NS-account for the details of the invoice. We cannot see it here. If you need help with the specifics, please get in touch with our customer service.

I received invoice for April 2023 but I noticed that half invoice included Mei, how it can be?

Monthly invoices don’t necessarily correspond to calendar months. If your subscription started halfway during the month, for example, then your invoices will always be for the last half of one month and the first half of the following.