How do you get this calculation to occur so a customers enters and sees total before paying?

how do you get this suggested calculation to occur in the form.  The customer enters the invoice amount, then there is a calculation to add a set online payment fee to achieve a total that the customer sees before they pay?

To do this what you need to do is create a form calculation to combine the total then you can have your paypal field get that user defined amount which can be shown to the customer before paying.

You can read more up on how to do this here in these guides below:

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

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field 

Here's a demo for you just to show you how you could do this as an example.

 

Demo Form -  https://www.jotformpro.com/form/70065207751956   [Clone It!]

 

I used a conditional calculation which can control the actions of other fields accordingly but, the form calculation widget is similiar just without that and does the same for your calculations so it is safe to use that as I mentioned at first if you wish.

In my demo I am using and created a calculation to get the Invoice Amount then multiply it by 2.9% + .30 which is the USD amount for paypal's fees in my location and the calculation does the rest.

I am combining these together in the second field below the invoice fee but, if you want to get fancy you could just show that on the second field and have a third to combine it if you like to show that beforehand.

That is then pulled into the Paypal Payment Integration on your form to show to the user so they know what they are paying in advance before submitting to finalize the transaction.

You can also choose to hide that field but for demonstration purposes I kept everything visible for it so that you could see how it's occurring.

thank you all.  I will study these and try to understand and may post back later with clarifying questions.  I really appreciate your help!