How do I calculate a $ amount without using a payment module

The form I'm building offers the user the option of paying by check or by credit card. Clicking the "Payment Method" radio button for credit card displays the payment info which works fine. How do I display a selection of 6 items that the user can select from (either one or more items) and have a calculation total up the price for each selection. (Each selection should equal $40.)

You first need to assign a value to each option: https://www.jotform.com/help/301-How-to-Assign-Calculation-Value 

Then, you can perform the calculation with the Form Calculation widget: https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget 

If you finally need to pass it to your payment module, here is how: https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field 

Let us know if you need more help, we will be glad to assist you.