How can I pass more resource IDs with a single request?
Implement hook_services_resources().
function example_services_resources() {
  $resources = array();
  $resources['einvoice'] = array(
    'actions' =&gt; array(
      'search' =&gt; array(
        'access callback' =&gt; '_example_resource_access',
        'callback'        =&gt; '_example_resource_search',
        'help'            =&gt; 'E-invoice online checker.',
        'file'            =&gt; array(
          'type'   =&gt; 'inc',
          'module' =&gt; 'exmaple',
          'name'   =&gt; 'resources/exmaple.resource',
        ),
        'args'            =&gt; array(
          array(
            'name'        =&gt; 'input',
            'optional'    =&gt; FALSE,
            'source'      =&gt; array('data' =&gt; 'input'),
            'type'        =&gt; 'string',
            'description' =&gt; 'Put the query string to get some data.',
          ),
        ),
      ),
    ),
  );
  return $resources;
}

In the example.resource.inc file, you put the functions declared  in that hook.
function _example_resource_access() {
  return TRUE;
}

function _example_resource_search($input) {
  // To do something you want.
}