Need add a feature to a purchased script from Code Canyon

I bought a script from code canyon and I need to add a little change about the service charge is added to the invoice. It's added a fixed amount. I need it 6% service charge added only if the Dine-in button is selected. and the calculated amount should be as (subtotal amount + 6% = Grand Total). Should continue to bill and invoice and reports. Developer should Need a fluent knowledge of Codeigniter. It's Urgent. The project goes to the lowest budget and good time management. Had to resubmit the Project due to time wasters.