Please post-up the NtO.
As it will say on the NtO, do not ignore it. Your original PCN will (or should) say something about if you made reps against the PCN but got a NtO without any response to the reps, you must follow the instructions on the NtO.
My advice is to just submit the same reps again, but this time add in that you did not receive a response to your informal challenge against the original PCN. This means that you missed the opportunity to pay the discounted penalty, and you request that they re-offer it.