Any advice on what to do?

I received a parking charge notice from UKCPS on the windscreen of my car which was parked in a co-op car park. The PCN is for £100 (£60 if paid within 14 days). for leaving the site. Apparently I was seen by the parking attendant parking up and walking off site. I appealed the charge on the grounds that we purchased good from the store on the day and that the parking attendant had ticked the Parking Charge Notice inappropriately as the tick was outside any of the boxes on the paperwork. UKCPS rejected my appeal on the grounds that there are signs which clearly state that persons using the site must stay on site for the duration of the stay? No photographic evidence has been provided! They have given me the option to pay or appeal to the IAS. What should I consider doing? Any advice gratefully accepted.