WooCommerce: Add surcharge to all transactions. From https://docs.woocommerce.com/document/add-a-surcharge-to-cart-and-checkout-uses-fees-api/

surcharge-function.php

/**
 * Add a 1% surcharge to your cart / checkout
 * change the $percentage to set the surcharge to a value to suit
 * Uses the WooCommerce fees API
 *
 * Add to theme functions.php
 */
add_action( 'woocommerce_cart_calculate_fees','woocommerce_custom_surcharge' );
function woocommerce_custom_surcharge() {
  global $woocommerce;

	if ( is_admin() && ! defined( 'DOING_AJAX' ) )
		return;

	$percentage = 0.01;
	$surcharge = ( $woocommerce->cart->cart_contents_total + $woocommerce->cart->shipping_total ) * $percentage;	
	$woocommerce->cart->add_fee( 'Surcharge', $surcharge, true, '' );

}