code: func (h *Client) Tax(tax float64) *Client {
  	h.tax = tax
  	h.taxSet = true
  	return h
  }
docs: // The total tax associated with the transaction. 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.