How to add the PayPal processing fee to the total amount calculation when users select to pay through PayPal?

I have a similar question:  For PayPal I am using a total generated in the form (with the widget).  The TOTAL is what the customer uses for either paying by check or paying via PayPal.  When they click the PayPal option it hides the Pay By Check option and kicks them to the PayPal section.  I need to take that total and add the PayPal fee (2.9% + $.30) to it so they can make their PayPal payment and we don't get stuck with paying the fee.  

How do I do this?

You can have one form calculation field performing your usual calculation for the total amount to be paid, and then another form calculation field that takes the value from the first calculation, and adds the fee amount to it:

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

Then, you would simply pass the value from this second form calculation to the PayPal payment field, so if PayPal is selected, the user will pay the amount with the added fee:

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

Upon checking out your forms to provide an example, I see that you have already achieved this on your forms:

If you need any further assistance, please let us know. Thank you!