Smart parking fine in Northern Ireland

[ATC110]

The person concerned parked their hire car at a retail park in NI and did not notice any private parking signs or time limits. The first they knew was when the car hire company emailed them with a notice of a traffic violation which is a Smart parking invoice. The hire company procedure is to pass on the details of the hirer so the fine can be reissued to them by Smart so all dealings are then between the hirer and Smart.

The hire company charge the hirer an administration fee for this process and claim to be unable to prevent this being charged without a cancellation notice from Smart.
There are multiple stores in the retail park but none of them own the carpark and are not prepared to contact Smart parking to get the fine cancelled and have apparently had problems themselves with staff getting parking invoices too.

The questions are as follows:

It appears fines issued by Smart are not enforceable in Northern Ireland according to posts on this forum - should the fine be ignored when it arrives?
The hire company has access to the hirers prepaid debit card and will attempt to charge the administration fee so the debit card holder has a balance of zero in their account and has blocked their card - when would it be safe to unblock the account ie for how long will the hire company attempt to debit the admin fee?

Thanks in advance