MRP simply means that price of goods cannot exceed the displayed price. GST is not levied on the basis of MRP. GST is levied on that price on which goods are actually transacted. GST is levied on invoice price. Suppose MRP of any goods is Rs. 100 and it is sold at Rs 80. GST shall be levied on Rs. 80. This is the law. Even if goods are sold at loss then also GST shall be levied on that sale price. This sale price can be questioned only on related party transaction and not otherwise.
Consumers and purchasers always negotiate before buying. MRP is not sacrosanct.