/**
   Page for adding/editing invoices.  When first opened, displays the user selection form
   Also checks that all proper tables and settings are stup.
  */
 function page_manage_invoice() {
     global $wpi_settings;
     WPI_Functions::check_tables();
     WPI_Functions::check_settings();
     if (isset($_REQUEST['wpi']['new_invoice']) || isset($_REQUEST['wpi']['existing_invoice'])) {
         include($wpi_settings['admin']['ui_path'] . '/manage_invoice.php');
     } else {
         include($wpi_settings['admin']['ui_path'] . '/blocks/postbox_user_selection_form.php');
     }
 }