What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
You can try using a Form Calculation widget as the final price with additional 3% on the transaction, you can then pass the value from the Form Calculation to your payment total field, here's the guide on that: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field.
Here's an example:
1. Here's what I added in my form:
-a text box named "Donate", this is where we will fetch the donation of the user.
-a form calculation widget, we will get the donation value + 3%
-I set up the payment method to fetch the value on the form calculation widget. You can follow the guide above.
2. This is how I calculated the donation + 3% in the form calculation widget.
3. In the payment wizard, I'll select the "Calculation" field to where to get the final amount.
I hope that helps. Do let us know if you need more information on this.