I decided on the evening of April 22 (Friday) to change my subscription from a basic subscription to a free weekend subscription. But that night, there seemed to be a problem with the ns official website, and I couldn't log in. I also consulted my friends, but they couldn't log in. so on the next saturday morning i called ns customer service to ask for her help, on the phone i was pretty sure i made my point - change to the weekend subscription, then the staff told me that the change was successful in the system . I went to a lot of places that weekend. But yesterday I saw that my bank account was debited for 129 euros and when I checked the details I found that I was still on the basic subscription.
I consulted the chatting customer service of ns, and the information I got was that it was noted in the ns system that I asked to change to the basic package that day. This is shocking to me, in fact it is impossible for me to make such a request under normal logic, because my account has been stuck on the basic plan for a few months. There's no way I could ask to switch to a basic subscription in one weekend and then travel for two days.

I don't know how to fix it now. Customer service said they didn't have a voice recording at the time.
I don't feel like I should pay this bill. This is much more than expected.