Can I have customers pay the specific amount they owe, which varies, for a product instead of set prices on the form?

I understand that you want to have custom amount in addition to the set prices of the product on the form. If so, it is possible to achieve this by using the calculation wizard and pass this field to payment wizard.

Please see the demo JotForm if that works for you.

http://form.jotformpro.com/form/50205331457951?

I used the calculation values property to set the values for checkboxes. Please refer to the guide below to learn assigning calculation values to checkboxes/radio buttons.

http://www.jotform.com/help/301-How-to-Assign-Calculation-Value

Also, the following guide that can help you with passing the calculation value to the payment field.

http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field

You may clone the demo JotForm to your account to see how it works. If you are referring to something else, please let us know. We will be happy to help.

Thank you.