To add payment option to your calculator, you need to add Payments field.

This field enables your users to calculate total price and instantly pay the calculated amount through PayPal or Stripe. Excellent example is Calculoid Pricing page. After successful payment, user receives payment confirmation email and you can track all the payments and their statuses under Submissions (Details > Submissions).

Payment field requires purchase of a E-commerce or High Traffic subscription plan. 

Additional information