Mandhana Industries / Being Human — wrong levy of gst

I (Shri neelu karwani) have purchased being human mens non-denim-chino-bhndc7515-ecru-96 under bill no. Cm/3678/17 dated 23.12.2017 in r-city mall, ghatkopar at being human showroom. The value of the said garment was rs. 2999/- and it was under 40% discount. So i got a total discount of rs. 1199.60 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 notice that while purchasing the clothing of the same brand (Being human) from another store i. E. Globus store pvt. Ltd. On 27.12.2017 at r-city. We got the discount without gst imposing on me. And on the same date i purchased another garment from being human store at r-city, ghatkopar under tax invoice bill no. Cm/3936/17 dated 27.12.2017 under 50% discount, but here the gst was levied on mrp. When asked about the same, they were little confused and told me that it is a rush hour due to salman khan birthday and suggested me to mail it.
Ii is widely publicised in media that gst is included in mrp based item. So why this injustice to me. Why there are 2 different rules for the same reputed brand in two different stores.

It is requested to look into the matter and give me gift voucher or inform me accordingly so that i can think of future course of action.
Thanking you,
Regards,
Neelu karwani.
([protected]) mail id: [protected]@gmail.com