code: func (r Billing_Invoice) GetExcel() (resp []byte, err error) {
  	err = r.Session.DoRequest("SoftLayer_Billing_Invoice", "getExcel", nil, &r.Options, &resp)
  	return
  }
docs: // Retrieve a Microsoft Excel spreadsheet of a SoftLayer invoice. You must have a Microsoft Excel reader installed in order to view these invoice files.