Problem to make billing

[Kushal Neupane]

Dear all, i am developing Resturat Management system. I made models for all. I made a static webpage for billing using ajax. I have beverage and dishes model. Now i need to develop, sn with is auto increment as per items, bevereage/dishes items to select and type, rate as per items, qty should type as i know it i can do, total which must be calculate as per qty * rate, sub total and discount with grand total as per table number. Can ayone help me out.