How to add a set fee to payment integration?

I have integrated Authorize.net within my form but I need to add a set fee so that whenever someone uses that form it charges a fee. Would I do that within the payment settings or do I need to contact Authorize.net to do that? 

I've cloned your form and I can see you're using the user defined amount payment type, you're also asking your user directly for the amount to be paid in the payment field. 

What I would suggest you is to perform some calculations, this way you will be able to get the desired payment amount and include the fee you need to charge. 

On this guide, you will find more details about how to perform calculations on your form: https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget 

As an example, here's what I did on your form. 

First, I added a number field: 

Then, I added the Form Calculation widget and performed some calculations: 

Here's how you may perform your calculations: 

To pass the calculation to the payment field, please check the following guide: https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field 

Here's a sample form with this applied: https://form.jotformpro.com/71006029101943 

If you have any question, let  us know.