Mandhana Industries / Being Human — wrong gst levy

Address:

Mumbai City, Maharashtra, 400010

I (Burhan patanwala) have purchased being human mens ink navy chino bhndc7515 under bill no. 2634 dated 30.01.2018 in uni blu, mazgaon at bluez retail co.. The value of the said garment was rs. 2999/- and it was under 40% discount. So i got a total discount of rs. 1200 which is mentioned on the said tax invoice. So i have to pay rs. 1800/- (Round off). But the store charged me rs. 2015/- after imposing 12% gst and i had to pay rs. 2015/-.
It is well known fact in mrp item gst is included, then why gst was wrongly imposed on mrp.
Also to bring to your kind...