Everything that's needed is in the NTO i.e. will instead of may.
OP, it sounds like jargon, and it is. But this is a parking contravention and one of the grounds of representation is 'procedural impropriety'. In short, if they don't follow prescribed procedure and materially depart from what 'must' be stated in any of the formal penalty notices - the PCN and NTO - then they are likely to lose at tribunal.
Notice to owner
20.—(1) Where—
(a)a penalty charge notice has been given with respect to a vehicle under regulation 9, and
.....
(3) A notice to owner must, in addition to the matters required to be included in it under regulation 3(2) of the 2022 Appeals Regulations, state—
......
(g)that if, after the payment period has expired, no representations have been made under regulation 5 of the 2022 Appeals Regulations and the penalty charge has not been paid, the enforcement authority may increase the penalty charge by the applicable surcharge,
The NTO states 'will' increase if payment or reps are not made. Apart from anything else this is contradictory nonsense procedurally because in the same NTO they state(under Representations) that if reps are made outside the 28-day period they 'may be disregarded'. You cannot 'will' increase at the trigger event if at the same time you only 'may' disregard.
By the way, in what form does the permit come i.e. do you provide the holder and adhesive or do they? It would be nice to counter their 'the motorist must adhere to regulations' with 'and your b****y permit must adhere to the windscreen'!