How do I create a donation form that lists pre-defined amounts and one user-defined amount

Subject line says it all. I need to create a donation form that enables the user to either select one of about 5 pre-determined amounts or enter their own user-defined amount. How do I do that?

Yes it is possible to achieve your requirement. Here are the steps you should follow:

#1. Instead of displaying the options in the PayPal question, you can add the products using a check box question and assign those options calculation values:

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

#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.

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget

#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 

That way, you can have whatever options you would like and include custom values.