woocommerce_shipstation_export_custom_field_2.php

<?php
/*
 * Send add-on field data to ShipStation
 * @link https://woocommerce.com/document/shipstation-for-woocommerce/
 */
// This is for custom field 3
function prefix_shipstation_custom_field_2() {
	return '_meta_key_2'; // Replace this with the key of your custom field
}
add_filter( 'woocommerce_shipstation_export_custom_field_2', 'prefix_shipstation_custom_field_2' );

// This is for custom field 3
function prefix_shipstation_custom_field_3() {
	return '_meta_key_3'; // Replace this with the key of your custom field
}
add_filter( 'woocommerce_shipstation_export_custom_field_3', 'prefix_shipstation_custom_field_3' );