Need to be able to use "Default - One Row Per Item" setting but not getting "admin custom fields" and not getting concatenated shipping_firstname + shipping_last_name ---- seems to work just fine with "Default"

end-resulting-modified-exporter.php

<?php
/**
 * Plugin Name: WooCommerce Modify CSV Exported file
 * Description: Adds and removes desired and undesired fields respectively
 * Version: 1.0.0
 * Author: Ray Flores
 * Author URI: http://www.rayflores.com
 * Requires at least: 3.9
 * Tested up to: 3.9
 *
 *	License: GNU General Public License v3.0
 *	License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */
 
 
// set the data for each for custom columns
function rf_wc_csv_export_modify_row_data( $order_data, $order, $generator ) {
 
	$new_order_data = array();
 
	$order_fields = $GLOBALS['wc_admin_custom_order_fields']->get_order_fields( $order->id );
 
	$custom_data = array(
		'cust_num'                 => is_object( $order_fields[3] ) ? $order_fields[3]->get_value_formatted() : '',
		'shipping_full_name'       => $order->shipping_first_name . ' ' . $order->shipping_last_name,
		'shipping_address_1'       => $order->shipping_address_1,
		'shipping_address_2'       => $order->shipping_address_2,
		'shipping_postcode'        => $order->shipping_postcode,
		'customer_message'         => $order->customer_message,
		'custom_tracking_provider' => '', // Add meta here
	);
 
	if ( 'default_one_row_per_item' === $generator->order_format ) {
 
		foreach ( $order_data as $data ) {
			$new_order_data[] = array_merge( (array) $data, $custom_data );
		}
	} else {
 
		$new_order_data = array_merge( $order_data, $custom_data );
	}
 
	return $new_order_data;
}
add_filter( 'wc_customer_order_csv_export_order_row', 'rf_wc_csv_export_modify_row_data', 10, 3 );
 
 
// set order columns
function rf_wc_csv_export_custom_order_columns( $column_headers, $generator ){
 
	if ( 'default_one_row_per_item' === $generator->order_format ) {
 
		$column_headers = array(
			'cust_num'                 => 'cust#',
			'order_date'               => 'date',
			'date_shipped'             => 'shipment_date',
			'blank'                    => 'blank',
			'custom_tracking_provider' => 'ship_via',
			'order_id'                 => 'order_id',
			'item_sku'                 => 'item #',
			'item_quantity'            => 'quantity',
			'shipping_full_name'       => 'First and Last name',
			'shipping_last_name'       => 'Last name',
			'shipping_address_1'       => 'Address1',
			'shipping_address_2'       => 'address2',
			'shipping_city'            => 'city',
			'shipping_state'           => 'st',
			'shipping_postcode'        => 'zip',
			'customer_note'            => 'customer notes',
		);
	}
 
	return $column_headers;
}
add_filter( 'wc_customer_order_csv_export_order_headers', 'rf_wc_csv_export_custom_order_columns', 10, 4 );

mod-csv-exporter.php

<?php
 /**
 * Plugin Name: WooCommerce Modify CSV Exported file
 * Description: Adds and removes desired and undesired fields respectively
 * Version: 1.0.0
 * Author: Ray Flores
 * Author URI: http://www.rayflores.com
 * Requires at least: 3.9
 * Tested up to: 3.9
 *
 *	License: GNU General Public License v3.0
 *	License URI: http://www.gnu.org/licenses/gpl-3.0.html
*/

// remove a column
function wc_csv_export_remove_column( $column_headers ) {
 
	// the list of column keys can be found in class-wc-customer-order-csv-export-generator.php
	unset( $column_headers['order_id']);
	unset( $column_headers['order_number'] );
	//unset( $column_headers['order_date']);
	unset( $column_headers['status'] );
	unset( $column_headers['shipping_total'] );
	unset( $column_headers['shipping_tax_total'] );
	unset( $column_headers['tax_total'] );
	unset( $column_headers['cart_discount'] );
	unset( $column_headers['order_discount'] );
	unset( $column_headers['discount_total'] );
	unset( $column_headers['order_total'] );
	unset( $column_headers['payment_method'] );
	unset( $column_headers['shipping_method'] );
	unset( $column_headers['customer_id'] );
	unset( $column_headers['billing_first_name'] );
	unset( $column_headers['billing_last_name'] );
	unset( $column_headers['billing_company'] );
	unset( $column_headers['billing_email'] );
	unset( $column_headers['billing_phone'] );
	unset( $column_headers['billing_address_1'] );
	unset( $column_headers['billing_address_2'] );
	unset( $column_headers['billing_postcode'] );
	unset( $column_headers['billing_city'] );
	unset( $column_headers['billing_state'] );	
 	unset( $column_headers['billing_country'] );
 	unset( $column_headers['shipping_company'] );	
 	unset( $column_headers['billing_country'] );
	unset( $column_headers['shipping_country']);	
	unset( $column_headers['_custom_tracking_link']);
	unset( $column_headers['tracking_provider']);
	unset( $column_headers['_date_shipped']);
	unset( $column_headers['tracking_number']);
    unset( $column_headers['customer_note'] );
	unset( $column_headers['shipping_first_name'] );
	unset( $column_headers['shipping_address_1'] );
	unset( $column_headers['shipping_address_2'] );
	unset( $column_headers['shipping_postcode'] );
	unset( $column_headers['shipping_city'] );
	unset( $column_headers['shipping_state'] );
	unset( $column_headers['shipping_country'] );	
	unset( $column_headers['shipping_last_name'] );
	unset( $column_headers['shipping_items']);
	unset( $column_headers['tax_items']);
	unset( $column_headers['coupon_items']);
	unset( $column_headers['order_notes']);	
	unset( $column_headers['order_currency']);
	unset( $column_headers['item_name']);
	unset( $column_headers['item_sku']);
	unset( $column_headers['item_quantity']);
	unset( $column_headers['item_total']);
	unset( $column_headers['item_meta']);
	unset( $column_headers['line_items']);
	

	
	return $column_headers;
}
add_filter( 'wc_customer_order_csv_export_order_headers', 'wc_csv_export_remove_column' );


function remove_filters_from_shipping_class( ) {
if (class_exists('WC_Shipment_Tracking')) {
global $WC_Shipment_Tracking; //globalize class from another plugin
				// filters from WC_Shipment_Tracking class - which add the tracking info into the exporter class - needed to remove entire filters
				remove_filter( 'wc_customer_order_csv_export_order_headers',   array( &$WC_Shipment_Tracking, 'add_tracking_info_to_csv_export_column_headers' ) );
			//	remove_filter( 'wc_customer_order_csv_export_order_row',       array( &$WC_Shipment_Tracking, 'add_tracking_info_to_csv_export_column_data' ), 10, 3 );
				}
}
add_action('plugins_loaded','remove_filters_from_shipping_class'); // after shipment-tracking plugin has loaded

function sv_wc_customer_order_csv_export_item_multi_column_headers( $column_headers, $generator ) {
 	$new_column_headers = array();
 	if ( 'default_one_row_per_item' === $generator->order_format ) {
 
		foreach ( $column_headers as $column_key => $column_header ) {
 
				$new_column_headers['cust_num']     = 'cust_num';
	 
			$new_column_headers[ $column_key ] = $column_header;
		}
 		$column_headers = $new_column_headers;
 	}
 	return $column_headers;
}
add_filter( 'wc_customer_order_csv_export_order_headers', 'sv_wc_customer_order_csv_export_item_multi_column_headers', 10, 2 );
 
// set the data for each for custom columns
function wc_csv_export_modify_row_data( $order_data, $order ) {
 $order_fields = $GLOBALS['wc_admin_custom_order_fields']->get_order_fields( $order->id );
 foreach( $order->get_items() as $_ => $item ) {

			$product = $order->get_product_from_item( $item ); }
	$custom_data = array(
		'cust_num' => is_object( $order_fields[3] ) ? $order_fields[3]->get_value_formatted() : '',
		'blank' => is_object( $order_fields[99999] ) ? $order_fields[99999]->get_value_formatted() : '',
		'item_sku'      => $product->get_sku(),
		'item_quantity' => $item['qty'],
		'shipping_full_name' => $order->shipping_first_name . ' ' . $order->shipping_last_name,
		'shipping_address_1'  => $order->shipping_address_1,
		'shipping_address_2'  => $order->shipping_address_2,
		'shipping_postcode'   => $order->shipping_postcode,
		'customer_message' => $order->customer_message
		
		// add other row data here in the format column_key => data
	);
 
	return array_merge( $order_data, $custom_data );
}
add_filter( 'wc_customer_order_csv_export_order_row', 'wc_csv_export_modify_row_data', 10, 2 );


// reorder columns
function wc_csv_export_custom_order_columns( $column_headers, $generator ){
 
$new_column_headers = array();
 
	if ( 'default_one_row_per_item' === $generator->order_format ) {
	
	foreach ( $column_headers as $column_key => $column_name ) {
		
		$new_column_headers[ $column_key ] = $column_name;
		
			$new_column_headers['cust_num'] = 'cust#';				
			$new_column_headers['order_date'] = 'date';
			$new_column_headers['date_shipped'] = 'shipment_date';
			$new_column_headers['blank'] = 'blank';
			$new_column_headers['custom_tracking_provider'] = 'ship_via';
			$new_column_headers['order_id'] = 'order_id';
			$new_column_headers['item_sku'] = 'item #';
			$new_column_headers['item_quantity'] = 'quantity';
			$new_column_headers['shipping_full_name'] = 'First and Last name';
			$new_column_headers['shipping_last_name'] = 'Last name';
			$new_column_headers['shipping_address_1'] = 'Address1';
			$new_column_headers['shipping_address_2'] = 'address2';
			$new_column_headers['shipping_city'] = 'city';
			$new_column_headers['shipping_state'] = 'st';
			$new_column_headers['shipping_postcode'] = 'zip';
			$new_column_headers['customer_note'] = 'customer notes';			
			unset( $column_headers['order_date']);

	}
	}

	return $new_column_headers;
}
add_filter( 'wc_customer_order_csv_export_order_headers', 'wc_csv_export_custom_order_columns', 10, 4 );

@rayflores Here's an updated and simplified Gist: https://gist.github.com/tamarazuk/c5a4caaef8c5f450d81d

Rather than unsetting the columns you do not want, I overwrote the column headings in rf_wc_csv_export_custom_order_columns(). This also means you don't need wc_csv_export_remove_column() or remove_filters_from_shipping_class().

I made bunch of other changes which are outlined here: https://gist.github.com/tamarazuk/c5a4caaef8c5f450d81d/revisions

thank you so much! this works... client wished to use shipping-tracking on the site, so, i used the remove filters from that plugin 👍