Donor option to pay bank fees

-- url of the page with the problem -- : angelwingsfdn.org
-- HikaShop version -- : 2.4.0
-- Joomla version -- : 3.4.1

Requesting best options to add "user/donor" option such as a radio button to calculate % in the event they would like to cover paypal or other fees. I would like an automatic solution whereas the user would:

1) fill in donation amount (-via custom field - works flawless thanks to Hikashop!)
2) allow the user to optionally select radio button to force a fee calculation for paypal use based on custom input field in donation.

Ex.
Donate $100 then user selects "Donor pay the banking fee" and then a calculation provided the solution on the cart. I have adapted to demonstrate in attached image.

How to do it? As a tax, as a shipping fee, or something without to much of a headache but looks slick

In the image the processing fee is predicated on the mandatory input on the paypal plugin. I want it to be shopping cart or product specific. A huge thanks for assistance and I will try and pay it forward as I am a learner!