As has been noted above - quite a few store car parks in UK realise that patrons object to paying a parking fee for a brief visit: their solution is to offer free parking for 90 minutes, then add a charge after that. One still has to take a ticket to prove time of entry to the car park.

 If you avail yourself of the free parking offered then you can not return within 2 hours - this stops people taking advantage of the offer provided and using the facility like a revolving door.