1. In the AbanteCart admin panel go to the Design > Email templates and add a new variable name to the Allowed Placeholders

  2. Search in the AbanteCart code for the required email template text_id

  3. Create a hook in the extension to add a new variable into the email template data

Public function onModelCheckoutOrder__ProcessData()
{
  $that = &$this->baseObject;
  $that->data['mail_template_data']['foo'] = 'bar';
}