Change the way of the calculation on POS php script

I want some changes on this script [login to view URL] I want the app to work in this way [login to view URL] of items is 190 so 190 * 156.67 =28.200.6 [login to view URL] sume of so 28.200.6 -30% =8.460.18 so 30% is custom discount who we give to diferent clients for diferent products,so i want the discount to be added to each product when we add products to POS invoice 3. Next step is 28.200.6 - 8.460.18=19.740 4. Next step is 19740*3% =592.20 5. Next step is 19740-592.20=19147.80 is the calcuation of -discount 1 and-discount 2 [login to view URL] step is 19147.80+18% VAT=3446.604 [login to view URL] step is 19147.80+3446.604=22594.404 8.22594.404 This is the final price the buyer need to pay This was an example of the calculation the system need to be updated to work in this way.