It may look like a fine, but it's really an invoice which they expect you to pay for agreeing to the terms and conditions of using their land. They can purchase you details via the DVLA which presumably explains the increase in admin costs for them and which is therefore passed onto you.
Must admit, pay and display car parks put me off going to some shopping outlets, especially since other stores within the chain are often placed in free car parks within a 15-20 minute drive.