Tasmac — Price of a bottle of beer

Address: Chennai, Tamil Nadu, 600041

I am living at Adyar, Chennai. In all the nearby Tasmac shops, They charge Rs. 10 or 20 or 30 more than the actual m.r.p written in that. It does not matter for me. But a bottle of pint beer which has an m.r.p as Rs.65/- they sell it for Rs.75/-.
When I wanted to buy a case of same beer (24 bottles) he says, it costs Rs.1800/-. (That is Rs.75 x 24 instead of Rs.65 x 24. ) My question is, why should I pay Rs.240 more when I buy as a case.