I suspect you just need this:
CREATE TABLE dbo.Orders (
    ORDER_NUMBER numeric(15, 2) NOT NULL CONSTRAINT [DF_Orders_ORDER_NUMBER] DEFAULT ((0)),
    ST_ID numeric(10) NOT NULL CONSTRAINT [DF_Orders_ST_ID] DEFAULT ((0)),
  TOTAL_PAYMENTS money NOT NULL CONSTRAINT [DF_Orders_TOTAL_PAYMENTS] DEFAULT ((0))
    CONSTRAINT [PK_Orders] PRIMARY KEY CLUSTERED
        (
        ORDER_NUMBER ASC
        )
    WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
);
GO

SET ANSI_PADDING ON;
GO

INSERT INTO dbo.Orders (ORDER_NUMBER, ST_ID, TOTAL_PAYMENTS)
    VALUES    (20650917.00,111156,220.50)
            ,(21888549.00,111156,220.50)
            ,(23475974.00,111156,245.00)
            ,(15438023.00,115240,97.00)
            ,(26580872.00,115240,166.00)
            ,(17064426.00,122994,117.50)
            ,(21887826.00,122994,121.00)
            ,(26467822.00,122994,130.00)
            ,(16240398.00,132943,117.50)
            ,(20445595.00,132943,172.50)
            ,(16694116.00,134513,245.00)
            ,(17437801.00,134513,220.50)
            ,(21784193.00,134513,247.50);

SELECT TOP(1) ST_ID, SUM(TOTAL_PAYMENTS) AS highest
FROM dbo.Orders
GROUP BY ST_ID
ORDER BY SUM(TOTAL_PAYMENTS) DESC;