code: func (r Billing_Invoice_Next) GetExcel(documentCreateDate *datatypes.Time) (resp []byte, err error) {
  	params := []interface{}{
  		documentCreateDate,
  	}
  	err = r.Session.DoRequest("SoftLayer_Billing_Invoice_Next", "getExcel", params, &r.Options, &resp)
  	return
  }
docs: // Return an account's next invoice in a Microsoft excel format.