Add Custom Var to Follow Up Emails.php

/**
 * Add Custom Variable
 */
add_action( 'fue_before_variable_replacements', 'fue_register_variable_replacements', 10, 4);

function fue_register_variable_replacements( $var, $email_data, $fue_email, $queue_item ) {
	$variables = array(
    	'tracking_number'	=> ''
	);

    if ( !empty( $queue_item->order_id ) ) {
		$order = wc_get_order( $queue_item->order_id );
		$tracking_number = get_post_meta( $order, '_wc_acof_2', true );
		$tracking_link = '<a href="https://www.osmworldwide.com/tracking/?trackingNumbers=' . $tracking_number . '">Track Delivery</a>';

		if ( !empty( $tracking_link ) ) {
			$variables['tracking_number'] = $tracking_link;
		}
	}

    $var->register( $variables );
}