public static function instance() {
		if ( is_null( self::$instance ) ) {
			self::$instance = new self();

			// Hook in actions once.
			add_action( 'woocommerce_checkout_billing', array( self::$instance, 'checkout_form_billing' ) );
			add_action( 'woocommerce_checkout_shipping', array( self::$instance, 'checkout_form_shipping' ) );

			// woocommerce_checkout_init action is ran once when the class is first constructed.
			do_action( 'woocommerce_checkout_init', self::$instance );
		}
		return self::$instance;
	}