code: public Redemptions getCouponRedemptionsByInvoice(final String invoiceId, final QueryParams params) {
          return doGET(Invoices.INVOICES_RESOURCE + "/" + invoiceId + Redemption.REDEMPTION_RESOURCE,
                  Redemptions.class, params);
      }
docs: Lookup all coupon redemptions on an invoice given query params.
  
  @param invoiceId String invoice id
  @param params {@link QueryParams}
  @return the coupon redemptions for this invoice on success, null otherwise