Why gst charged on mrp

Hi Team, I am a online seller which sold units through flipkart. I have received a email by flipkart and they was told : Let us consider that the MRP of a shoe is Rs. 700 and the selling price is Rs. 400, then the tax slab will be 18% (MRP above 500). if the MRP of the shoe is 499 and the selling price is Rs. 400. Then, tax slab will be 5% (MRP< 500). Team, I want to ask you that statement is right or wrong ?