A bought a biscuit packet of mrp 20 but the retailer is selling it at 30 rupees

I purchased a biscuit packet from a shop at the railway station. The packet had a printed price of Rs. 20, but I was charged Rs. 30 for it, which is Rs. 10 more than the maximum retail price. I am raising this complaint as this amounts to overcharging by the shop beyond the printed MRP. I request that the shop be directed to refund me the excess amount charged and be warned against overcharging customers in future.