Brand Factory — wrong billing

I have purchased clothes worth rs2999 (Mrp) on 40% discount
But they charged 5% extra (Rs90) on the discounted amount of rs 1800
This 5% was charged other than gst of 12%
And to my question they replied that it is the rule to charge 5% extra on any clothing article that is on discount
So this 5% is unfair as it is only going in the company's pocket and not to the government or the consumer