For this price (in 2022) you get OFF-PEAK travel only-that means ONLY check-INs in the off peak hrs do not charge extra. Everyone with account can get access to mijnNS and see what is registered (that should be no surprise as such) so you can easily check all by yourself. First do that, then come back if it remains unclear.

The most common errors are:

#check in in peak hrs. Full fare will be charged. There is nothing you can now do against that.

#forget to properly check in/out (this will cost 20 or even 50€ extra)-can mostly be rectified.

It is always very wise in a country where you are foreigner to first very thoroughly check how things work and what one should do or not do.