Showing them to who? None of our car parks are staffed. @Anotherjohn is right: the local authority car park beside Sainsbury's in Eltham (ugh) does it that way: a two-part ticket. Sainsbury's is a big enough retailer it can easily take the hit given that so many of the parked cars are for Sainsbury's. For the Perry Vale car park, the retailers/restaurants nearest to the car park will bear the 'hit' of refunding the parking charges.
Easiest thing to do, I suspect, is to reprogramme the machines for 2 hours free parking.