Price assignment. Form fields can have prices assigned to them. You can thus associate items with different prices or specify a fixed amount that will be charged at form submission.
To associate a certain product that you sell with a price, choose the form field that refers to the product in the dropdown list, press Add and type in its price.
Advanced calculations. Use calculation formulas to multiply a specific product by another field (e.g. Quantity) and specify taxes and discounts. Click on the Add a new calculation formula button to add another row to your list of formulas. Specify, if that is the case, taxes and discounts on total and mention what they represent.