code: public Redemption getCouponRedemptionByInvoice(final String invoiceId) {
          return doGET(Invoices.INVOICES_RESOURCE + "/" + invoiceId + Redemption.REDEMPTION_RESOURCE,
                  Redemption.class);
      }
docs: Lookup the first coupon redemption on an invoice.
  
  @param invoiceId String invoice id
  @return the coupon redemption for this invoice on success, null otherwise