/**
* Apply 3% + 0.30 cent fee to total, on the checkout page
*/
add_action( 'woocommerce_cart_calculate_fees', 'woocommerce_custom_surcharge' );
function woocommerce_custom_surcharge( $cart ) {
if ( is_admin() && ! defined( 'DOING_AJAX' ) )
return;
$percentage = 0.03;
$fixed_fee = 0.3;
$percentage_fee = ( $cart->cart_contents_total + $cart->shipping_total ) * $percentage;
$surcharge = $fixed_fee + $percentage_fee;
$cart->add_fee( 'Processing Fee', $surcharge, true );
}