Stored Procedure Newbie needs help!

[b]Trying to recalculate invoice items to update a parent table with totals -- not my design (ugh)

Here's my attempt at creating a stored procedure:


USE [Data1]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

CREATE PROCEDURE UpdateInvoiceHeader
      
      @InvoiceID int

AS
BEGIN
      
      DECLARE @InvLineItems FLOAT
      DECLARE @InvTotal FLOAT
      DECLARE @InvPayments FLOAT
      DECLARE @InvPOF FLOAT
      DECLARE @InvTaxPct FLOAT
      DECLARE @InvTaxAmt FLOAT
      DECLARE @InvBalanceDue FLOAT
      DECLARE @InvMgrDiscount FLOAT
      DECLARE @InvShipping FLOAT
      
      SET @InvLineItems = (SELECT Isnull(SUM(ISNULL([Qty],1)*[Price]) ,0) FROM InvoiceLines WHERE invoiceID = @InvoiceID)
      
      SET @InvPayments = (SELECT Isnull(SUM([PmtAmt]) ,0) FROM DepositLines WHERE invoiceID = @InvoiceID)
      
      SET @InvPOF = (SELECT Isnull(SUM([PmtAmt],0) FROM DepositLines WHERE InvID = @InvoiceID)
            
      UPDATE InvoiceHdr
      SET
      
            InvSubTotal            = @InvLineItems,
      
            Pmts                  = @InvPayments
            
            SET @InvTaxPct      = Isnull(TaxPercentage,0)      
            
            if NOT ISNULL(@InvTaxPct)
                  begin
                        SET @InvTaxAmt      = ROUND(convert(real,@InvLineItems) * Convert(real,(@InvTaxPct) / 100),2)
                  end
            else if
                  begin
                        SET @InvTaxAmt      =      0
                  end
            
            SET @InvMgrDiscount      = Isnull(MgrDisc,0)
            
            SET @InvShipping      = Isnull(Shipping,0)
            
            SET @InvTotal            = @InvLineItems + @InvTaxAmt + @InvShipping
            
            SET @InvBalanceDue      = @InvTotal - @InvPayments - @InvMgrDiscount
      
            TaxAmt                  = @InvTaxAmt
            BalanceDue            = @InvBalanceDue
      
      Where InvoiceID            = @InvoiceID      

END


Here are the errors I'm getting: There are far fewer than when I began :)


Msg 174, Level 15, State 1, Procedure UpdateInvoiceHeader, Line 24
The SUM function requires 1 argument(s).
Msg 174, Level 15, State 1, Procedure UpdateInvoiceHeader, Line 35
The isnull function requires 2 argument(s).
Msg 156, Level 15, State 1, Procedure UpdateInvoiceHeader, Line 39
Incorrect syntax near the keyword 'else'.
Msg 102, Level 15, State 1, Procedure UpdateInvoiceHeader, Line 52
Incorrect syntax near 'TaxAmt'.