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You must be referring to the Custom Payment Form.
Please check this user guide as well.
user guide: How-to-pass-a-calculation-to-a-payment-field
You can test the demo form http://www.jotformpro.com/form/41413741780956
The important part is the use of Calculation Widget and then passing the calculated value to the existing payment tool (i.e. Paypal) in the form.
Hope this help. Let us know how we can be of further assistance.