When buying a certain number they give a free service
Question

                What is the ruling on some stores, such as gas stations and laundries, giving their customers cards which the customer collects, then when he reaches a certain number they will do some service for him for free?
Praise be to Allah. 
          With regard to what some gas stations 
          and stores do, giving their customers gifts such as handkerchiefs or 
          cards to collect for a car-wash or oil-change, or paying a certain amount 
          up front for a discount on each wash, or paying some amount in advance 
          – I do not think that there is any reason not to allow such dealings, 
          because that is like a person giving up some of what belongs to him. 
          I mean the owner of the gas station, laundry or store. It is not like 
          gambling because the person who engages in these three transactions 
          does not lose anything. What he pays is in return for something and 
          any rewards or gifts that he gets are like incentives to do business.