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I am not sure if I have understood your question correctly. Do you want to actually add a text box in the payment question itself?
Unfortunately, it is currently not possible to achieve your requirement of adding additional text box in payment question.
one possibility, I can think of is to add normal check box question and textboxes to show products in your form and then you can add "Form Calculation" widget to calculate the amount and pass on it to the payment question of your form. The following guide should help you on how to pass the calculated amount to payment question: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field
Hope this helps.
Do get back to us if you meant to ask something else.