Snippet to add invoice number in fedex label. Support for WooCommerce PDF Invoices By Bas Elbers. Fedex plugin - https://www.xadapter.com/product/woocommerce-fedex-shipping-plugin-with-print-label/

functions.php

/**
 * Snippet to add invoice number in fedex label. Support for WooCommerce PDF Invoices By Bas Elbers.
 * Created at : 18 April 2018
 * Updated at : 18 April 2018
 * Xadapter Plugins : https://www.xadapter.com/shop/
 * Gist Link : https://gist.github.com/xadapter/79d628b412a2c7f9b3cbbcee419239a4
 */
add_filter( 'wf_fedex_request', 'xa_fedex_add_invoice_number', 10, 2 );

if( ! function_exists('xa_fedex_add_invoice_number') ) {
	function xa_fedex_add_invoice_number( $request, $order ) {

		$invoice_number = $order->get_meta('_bewpi_invoice_number');			// _bewpi_invoice_number to support WooCommerce PDF Invoices By Bas Elbers
		if( ! empty($invoice_number) ) {
			foreach( $request['RequestedShipment']['RequestedPackageLineItems'] as $key => $request_package_line_item ) {
				$request['RequestedShipment']['RequestedPackageLineItems'][$key]['CustomerReferences'][] = array(
					'CustomerReferenceType'	=> 'INVOICE_NUMBER',
					'Value'					=> $invoice_number,
				);
			}
		}
		return $request;
	}
}