code: func (h *Client) TransactionID(transactionID string) *Client {
  	h.transactionID = transactionID
  	h.transactionIDSet = true
  	return h
  }
docs: // The transaction ID. This is an additional parameter that
  // can be sent when Product Action is set to 'purchase' or
  // 'refund'. For analytics.js the Enhanced Ecommerce plugin
  // must be installed before using this field.