Use the Summary and Discounts section to add items to the payment summary of your order form, multiple your items with a Quantity, add discounts and also markups. The total that will be displayed in the Payment Summary table represents the amount that form users will have to pay you after submission.
Upon customizing the Payment Summary table, you can change the label names of your items (as shown in the summary), show the quantity value for each selected item and the detailed formula (if used).
Formulas are the ones you manually set up under the Assign Values or Formulas to Fields subsection, by using Number fields on your order form.
If you are using another version, check out this article with the same subject.