We had one of those jars at my local filling station, but the system got abused and had to be stopped. One bloke, we'll call him Dave, used to consistently fill up to £20.02 and take a 2p out of the jar each time. He was a regular visitor and on more than one occasion the jar was left completely empty. Whenever I saw him in the village, he was always sat in his car and always had a big grin on his face. It was estimated Dave made £2.36 over the five years that jar was in situ.