Snippet to update PO Number, Invoice Number and Recipient Tax number using WooCommerce FedEx Shipping Plugin with Print Label - https://www.pluginhive.com/product/woocommerce-fedex-shipping-plugin-with-print-label/

functions.php

/**
 * Snippet to update PO Number, Invoice Number and Recipient Tax number.
 * Created at : 30 July 2018
 * Updated at : 06 Aug 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/xadapter/f471f8266b21d68720c4d4d474e468bd
 */
add_filter('wf_fedex_request','wf_fedex_label_request_reff_callback', 10, 2 );
function wf_fedex_label_request_reff_callback($request,$order){
    $po_number = $order->get_meta('customer_po'); 	// Give Purchase Order here
	$inv_number = $order->get_order_number(); 	// Give Invoice number here, x if you don't want
	$tax_number = $order->get_meta('billing_abn'); 	// Give Tax number here
	$customs_duties_payer = 'SENDER'; 				// Give $customs duties payer here
	$tin_type = 'PERSONAL_STATE';					// Tax Identification number type BUSINESS_NATIONAL, BUSINESS_STATE, BUSINESS_UNION, PERSONAL_NATIONAL, PERSONAL_STATE
    
    $reff = array();
    if( !empty($po_number) && $po_number!='x' )
    	$reff['CustomerReferences'][] = array( 'CustomerReferenceType' => 'P_O_NUMBER', 'Value' => $po_number );
    
    if( !empty($inv_number) && $inv_number!='x' )
    	$reff['CustomerReferences'][] = array( 'CustomerReferenceType' => 'INVOICE_NUMBER', 'Value' => $inv_number );
    
    $reff['CustomerReferences'][] = array( 'CustomerReferenceType' => 'CUSTOMER_REFERENCE', 'Value' => $order->id );
    
    if( !empty($customs_duties_payer) )
    	$reff['CustomerReferences'][] = array( 'CustomerReferenceType' => 'DEPARTMENT_NUMBER', 'Value' => 'Bill Duties : '.$customs_duties_payer);
    
    foreach ($request['RequestedShipment']['RequestedPackageLineItems'] as $key => $value) {
    		$request['RequestedShipment']['RequestedPackageLineItems'][$key] = array_merge($request['RequestedShipment']['RequestedPackageLineItems'][$key], $reff);
	}
	$request['RequestedShipment']['Recipient']['Tins'] = array(
		'TinType'	=>	$tin_type,
		'Number'	=>	$tax_number,
	);
	// To update in Commercial Invoice
	if( ! empty($request['RequestedShipment']['CustomsClearanceDetail']['CommercialInvoice']) ) {
		if( ! empty($inv_number) ) {
			$request['RequestedShipment']['CustomsClearanceDetail']['CommercialInvoice']['CustomerReferences'][] = array(
				'CustomerReferenceType'	=>	'INVOICE_NUMBER',
				'Value'				=> $inv_number,
			);
		}
		if( ! empty($po_number) ) {
			$request['RequestedShipment']['CustomsClearanceDetail']['CommercialInvoice']['CustomerReferences'][] = array(
				'CustomerReferenceType'	=>	'P_O_NUMBER',
				'Value'				=> $po_number,
			);
		}
	}
    return $request;
}