code: public function addConfiguration(ArrayNodeDefinition $builder)
      {
          parent::addConfiguration($builder);
          
          $builder->children()
              ->scalarNode('merchant_id')->isRequired()->cannotBeEmpty()->end()
              ->scalarNode('merchant_password')->isRequired()->cannotBeEmpty()->end()
              ->scalarNode('receiver_email')->isRequired()->cannotBeEmpty()->end()
              ->booleanNode('sandbox')->defaultTrue()->end()
              ->scalarNode('sandbox_url')->end()
          ->end();
      }
docs: {@inheritDoc}