// Add this code to your theme functions.php file or a custom plugin
add_filter( 'woocommerce_shipstation_export_custom_field_2', 'shipstation_custom_field_2' );

function shipstation_custom_field_2() {
	return '_meta_key'; // Replace this with the key of your custom field
}

// This is for custom field 3
add_filter( 'woocommerce_shipstation_export_custom_field_3', 'shipstation_custom_field_3' );

function shipstation_custom_field_3() {
	return '_meta_key_2'; // Replace this with the key of your custom field
}