vat amount charged but not shown in the bill

i bought a bed sheet from home town diamond plaza kolkata. its total price is Rs. 1595 and after discount the amount is Rs. 799. But after billing its showing 760 and then total amount i need to pay is 843. Now after i ask the concerned person about the extra amount charged he told me that 5% is charged as vat. But in the bill its not shown at all that vat is taken. I asked for the explanation but the person told me that its a policy of home town, they didnt show it in the bill. But my question is when i pay something i need to know the exact amount and it should be on the bill. They are confusing the customers and charges extra money without any prove in the bill.