Ah sorry about the misunderstanding, I thought the "TOTAL" cell was giving you the wrong value, I didn't realise that you were referring to the "Calculate Total" button.

In that case, change the subroutine to reset the purchase order (if you decided to use it) to 
Private Sub ResetPO()
    Dim i As Integer
    
    If Sheet4.Range("Shipping").Row > 19 Then
        For i = 19 To Sheet4.Range("Shipping").Row - 1
            Sheet4.Rows(i).EntireRow.Delete
        Next i
    End If
    
    Sheet4.Range("A15:K18").ClearContents
    
    Sheet4.Range("VendorName").ClearContents
End Sub
and change the Calculate Total subroutine to 
Sub PO_CalcTotal()
    Dim i As Long
    
    For i = 15 To Sheet4.Range("Shipping").Row - 1
        Sheet4.Cells(i, 11).Formula = "=A" & i & "*J" & i
    Next i
    
    Sheet4.Cells(Sheet4.Range("VendorName").Row - 1, 11).Formula = "=SUM(K15:K" & Sheet4.Range("VendorName").Row - 2 & ")"
End Sub
Hope this works now