Add code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s functions.php file as this will be wiped entirely when you update the theme.
Add a percentage based surcharge to all transactions
https://gist.github.com/woogists/f24ba17b9fdc0023cd72c95520f80723
Add a standard $ value surcharge to all transactions
https://gist.github.com/woogists/434bace784f8457f75706ea51ad711e5
Add a surcharge based on the delivery country
https://gist.github.com/woogists/cbd29812339d7cac083896e0c30cb8de