woocommerce条件字段

Question

当用户在woocommerce结帐中选择一个字段时，我无法弄清楚如何激活该功能，这是我的代码， 我希望它仅在用户在结帐选择字段中选择1时才处于活动状态

add_action( 'woocommerce_cart_calculate_fees','conditional_add_fee' );
function conditional_add_fee() {
     global $woocommerce;

     if ( is_admin() && ! defined( 'DOING_AJAX' ) )
          return;
     $subtotal = $woocommerce->cart->subtotal;
     $fee_percentage = 0.22; // 22 percent
     $fee = number_format( round( $subtotal * $fee_percentage, 2 ), 2 ); // rounded to two decimal places to match currency formatting
     $woocommerce->cart->add_fee( 'IVA', $fee, true, 'standard' );
}

感谢