Make sure you're prepared. Do not argue the toss about 'commercial justification', you'll be talking to someone from the store who's probably been given a very sketchy briefing.
So, normal pleasantries such as thanks for calling, sorry to bother you, I don't know how much you've been told etc...
And get your facts prepared and written down so you can succinctly put them across:
A few weeks ago I parked in your car park. I had my wife with me and as she doesn't trust me to do the shopping she went into your store while I stayed in the car. She spent about £***, for which I still have the receipt, and this took about 15-20 minutes.
When she returned she put the shopping in the boot while I went to pay the parking tariff which was £1. We then left.
A week later I received a notice from Excel parking that I owed them £100, apparently for not buying my ticket soon enough after parking, something they call 'exceeding the consideration period'.
We parked, we shopped and we paid for the time we were parked(in fact twice the length of time we were parked). I thought that I would make these points to Excel in response to their claim which I did, but it fell upon very deaf ears. It would seem that Excel are only concerned with their financial interests and not those of customers or Iceland and the reputational harm their actions could cause, given that ultimately they're threatening to take customers to court.
Speaking to you as someone who shops at Iceland regularly, I cannot think that you would want to be associated with this form of practice.
What help can you give me?
Would be my approach.
By the way, where is the sign - 'at the back we have a car park which is available for up to 1 hr', it's not visible in GSV. Was it an Excel sign e.g. with its logo etc?