A pilón is the small extra that a Mexican vendor adds to a purchase, such as one more quesadilla. An article in the journal of the Autonomous University of the State of Mexico defines it as “una cantidad extra de mercancía que el comerciante regala a su cliente en agradecimiento por su compra,” an extra quantity of goods that the vendor gives the customer in thanks for the purchase.
The dictionary of the Royal Spanish Academy lists a Mexican verb for the other side of the relationship. Amarchantarse means “hacerse cliente habitual de un comerciante,” to become a regular customer of a merchant.