OP, NTO posted Tue. 6 June.
Latest date of 28-day period commencing on date of service, namely 8 June, is 4 June.
You have a NTO so you have to either pay or make reps. You might as well make reps, but with what is in play evidentially you don't have a procedural leg to stand on - as per Incandescent- so IMO this should set the tone of your reps.
If they reject and re-offer the discount, then you have a choice and are better off than now where the full penalty is in play;
If they reject, then you still have a choice and financially are no worse off than you are now;
If they accept, then you're quids in.
So, what mitigation could you put forward or was this simply the driver chancing their arm?