Paypal additional fee (Amount Base)

function my_em_add_fee($paypal_vars, $EM_Booking, $EM_Gateway_Paypal){
	$prix = $EM_Booking->get_price_pre_taxes();
     $pourcentage = 5;
	$amount = $prix - ($prix * ($pourcentage/100));
	if ( !empty($amount) ){
		$itemcount = (count($EM_Booking->get_tickets_bookings()->tickets_bookings) + 1);
		$paypal_vars['item_name_'.$itemcount] = wp_kses_data("Frais paypal");
		$paypal_vars['quantity_'.$itemcount] = 1;
		$paypal_vars['amount_'.$itemcount] = $amount;
	}
	return $paypal_vars;
}
add_filter('em_gateway_paypal_get_paypal_vars','my_em_add_fee',1,3);