Opencart - Invoice Customization - VQMOD

Hi, We are using Opencart for E-commerce. Currently We are facing Calculations issues into the Invoices. Below are the Examples. For Example - One Product A is Costing Rs. 500 Inclusive of Taxes (5%) shown in open Cart Front End. Suppose The Customer Used a Coupon Code of 18% Discount on it. So The Price Comes Rs.410 Inclusive of Taxes (5%). Now What i need is Invoice Should Show The Gross Price (Rs.390) + Tax (Rs.19.5 Rounded off to Rs.20) = Total (Rs.410) The Above Calculation Should also work when more than Two items are purchased and Coupon is Applied only on One Product. Example is given Below For Example - Another Product B Cost is Rs. 610 inclusive of Tax (5%) and Customer has added along with Product A (See above example) So The Final Invoice Should Show the Calculations Perfectly Like This The Gross Price (Rs.390) + Tax (Rs.19.5 Rounded off to Rs.20) = Total (Rs.410) The Gross Price (Rs.581) + Tax (Rs.29.05 Rounded off to Rs.29) = Total (Rs.610) ============= Grand Total - Rs.1020=00