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Though it is not possible to have both products and a custom amount option in the payment field, it is possible to use a workaround to include both in the form and pass that value to the payment field.
#1. Instead of displaying the membership options in the PayPal question, you can add the products in a check box question. You can assign those products calculation values:
#2. Add a text box to collect the donation amount.
#3. Add a "Form Calculation" widget in your form to calculate the total amount of membership and donation.
#4. Add PayPal payment question in your form and set it to read the amount from the calculation widget. The following guide should help you how to pass calculation to PayPal payment question: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field
Please check the screenshot below for all the steps: