When i create Coupon buy 1 get 1 free. I use Coupon that works fine and show the right total bill but when i create regular invoice then on the invoice  i see the actual price which is without coupon. for example product name bag and the price of bag = $15.
i create quotation and fill all the necessary information then set quantity of bag = 2 after this i use coupon code that works fine the total comes on the sale order that is $15 because i use coupon 2 bag = 30$ - 1 Free bag  = total bill 15$. after that when i create invoice i saw price of bags = 30$.
can anyone guide me what's wrong there? where i'm doing mistake or is this bug ?