Shoppers Stop — billing discrepancy

Address:

Gurgaon, Haryana, 122001

I bought 4 products today. 2 @ 60% discount and 2 @40% discount.
After billing when i checked receipt, i realized items supposed to be at 40% discounts were charged more (With discount of ~35)
After checking with at customer service desk, their executive told there is 5% gst added. Which was not mentioned on bill for those items. After logical argument and that guy checked in system he acknowledged it was an system issue.
Further i thought of checking entire bill carefully then i realized 2 products which were at 60% also were given only 50% discount in bill. After further arguement with the shopper stop personal he checked in system and confirmed yes it shuld be 60% and miscalculation due to system error.
With shopper stop operating for ages, it system setup long long ago.. Still they make such miscalculation (Dont know intentionally) in name of system error. Assume 5-10% of access amount if they charge like this.. Assume their level of chori on middle man's pocket.