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Unfortunately, it's not possible to accept custom donations and have a product list on the payment tool directly. However, it's possible to accomplish this using normal fields.
On this cloned version of your form: http://form.jotformpro.com/form/50656448909972 I've implemented this using the steps provided in this guide: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field
Please check the form and if this is what you're looking to accomplish, feel free to clone it to your account to check it further.