Vtiger custom invoice module

Hello, I would like to custom Vtiger 6.2 invoice. with pdf maker or other. We would like to change php code to have an additional calculate for exemple; I driver drive on the day , customer pay 10$ + vat If the driver drive after [login to view URL] , the driver must add 35% supplement so 10$ *1.35(35%) + vat layout invoice must appear for example Total vat excl : 10$ extra night vat exl : 3.5$ Total vat excl : 13.5$ vat % = 21% Total vat incl: 16.45 More detail when project accepted