MRP cost and actula cost charged

I have purchased 3 Pepsi (200 ml) – which MRP Rs 8 total cost should be Rs.24 but shop owner (Sunday Treat) charged me Rs. 45 for which I have the bill bu I have argued for this cost difference of MRP
but he denied the same.
I dont understand how these peoples can charge the amount excess to MRP.
Shop addrss 14 Sarudhi Commercial, Opp Mind space, Chincholi bunder rd, link road Malad west Mumbai 64 phone no of Shop owner 9870581014 / 28729499