Private Sub Command58_Click()
    CalculateTotal
End Sub

Private Sub ProductId_Change()
    CalculateTotal
End Sub

Private Sub Amount_Exit(Cancel As Integer)
    CalculateTotal
End Sub

Private Sub CalculateTotal()
    Forms![frmCustomerInvoice]![Total] = DSum("[Amount]", "qryInvoiceItems", "[InvoiceNo] = Forms![frmCustomerInvoice]![InvoiceNo]")
End Sub