When buying a certain number they give a free service

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.