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I am not sure if U have understood your question correctly. Do you mean to say that you want to add an amount to the total of the payment question?
Yes it is possible to achieve your requirement. Instead of adding the products in the payment question itself, you will have to use normal form fields to list your product. You can then add "Form Calculation" widget in form to calculate the amount based on the selection of products which can be passed on to payment question of form.
You may like to take a look at the following guide and see if that helps you: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field
Please take a look at the following demo form I created for you : http://form.jotformpro.com/form/52770970169969? Feel free to clone this form for a closer look. The following guide should help you in form cloning: http://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL
You may like to check the following screenshot as well for all the steps:
Hope this helps.
Do get back to us if you have any questions.