code: func (h *Client) CouponCode(couponCode string) *Client {
  	h.couponCode = couponCode
  	h.couponCodeSet = true
  	return h
  }
docs: // The transaction coupon redeemed 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.