We are a nonprofit that accepts donations through Paypal. Currently, when we receive a donation, it is the donation amount minus fees. We would like to add the option for donors to donate a little extra to cover the fees Paypal charges. Ideally, this would be an optional checkbox under their desired donation amount that adds a percentage to their total donation. Our website was made through nationbuilder, and we just need someone to add some code to our theme. Thank you.