How can I add formulas to add items for a total?

You can do that using the form calculation widget, here's the guide for that: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form.

You can also use the option "update a form field" under conditional logic to pass values to fields: http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic. 

If you are creating an order form and want to pass your calculation to your payment total field, then this guide might help you: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field.

Do let us know if you need more help on this.

Thank you.