Most of orders contain 1-2 items, but there could be 10+ and it becomes a problem to follow the invoice and picking items from the shelves. Any field of your choice: model, sku, etc. Here is an example how to update invoice and packing list (vqmod format):
<?xml version="1.0" encoding="utf-8"?>
<modification>
<id>For OpenCart</id>
<version>3.0.3.2</version>
<vqmver>2.5.1</vqmver>
<author>alex cherednichenko</author>
<link><![CDATA[https://www.alcher.info]]></link>
<file name="admin/controller/sale/order.php">
<operation error="skip">
<search position="before" index="2,3">
<![CDATA[
$data['orders'][] = array(
]]></search>
<add>
<![CDATA[
// alcher
array_multisort(array_map(function($element) {
return $element['model'];
}, $product_data), SORT_ASC, $product_data);
// alcher
]]>
</add>
</operation>
</file>
</modification>
Source: 3 solutions for sorting multidimensional arrays by child keys or values in PHP
