public function set_invoice_number() {
		$invoice = $this->get_invoice_number();

		// set the cookie (if it was already set, it'll extend the lifetime)
		$secure = 'https' === parse_url( home_url(), PHP_URL_SCHEME );
		setcookie( self::$invoice_cookie_name, $invoice, time() + $this->invoice_expiration_time, COOKIEPATH, COOKIE_DOMAIN, $secure );
		set_transient( $this->invoice_transient_name( $invoice ), '1', $this->invoice_expiration_time );

		return $invoice;
	}