Add Purchase Order ( PO ), Invoice, Customer Reference, and Department numbers

function.php

add_filter('wf_fedex_request','wf_fedex_label_request_reff_callback', 10, 2 );
function wf_fedex_label_request_reff_callback($request,$order){
    $po_number = 'x'; //give Purchase Order here
    $inv_number = 'x'; // Give Invoice number here
    $customs_duties_payer = 'SENDER'; // Give $customs duties payer here
    
    $reff = array();
    if( !empty($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);
    }
    return $request;
}