Over charging above MRP

My son went to the grocery shop to buy a pepsi cold drink bottle. Mrp was rs 18 but the shop owner sold it for 30 rs.. and when my son questioned him, he said u can keep the bottle as well.. my son refused to keep the bottle and asked him to cut 18 rs only.. then harshfully and rudely he replied, bottle is for 30 only . U want to buy to the buy or else u can leave . Shop owner is forcefully charging 30 rs for a 18 rs cold drink bottle. Even i went to talk with him and he started fighting with me and talk rudely as well.. i have a video proof as well.. and i also have that bottle with rs 18 mrp printed on it .