Upbhokta Suvidha Kendra — Charging higher price than MRP

All Provisional stores nr GCC in mira road charging one rs. extra for packed milk as a cooling charge. They c harge this for all brands like amul and mother dairy. When we ask for written invoice or bill they flatly deny.. They say all these companies are charging 1rs extra to them. Kindly suggest course of action to avoid this.