Add event to customize product weight

shop_ordertitem.php

<?php 

public function total_weight()
{
	$result = $this->om('weight')*$this->quantity;
	
	$extras = $this->get_extra_option_objects();
	foreach ($extras as $option)
		$result += $option->weight*$this->quantity;
	
	$weight_adjusted = Backend::$events->fire_event(['name' => 'shop:onGetOrderItemWeight', 'type'=>'filter'], $this);
				
	return ($weight_adjusted) ? $weight_adjusted : $result;
}