If AllowanceTxtBox.Text <> "" Then
                Try
                    Integer.Parse(AllowanceTxtBox.Text)
                Catch AllowanceError As Exception
                    MessageBox.Show("You didn't input numbers for the allowance price!", "Data Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
                    AllowanceTxtBox.Focus()
                    AllowanceTxtBox.SelectAll()
                End Try
            Else
                AllowanceTxtBox.Text = "0"
            End If
            AmountDueTxtBox.Text = FormatCurrency(CStr(CDbl(TotalTxtBox.Text) - CDbl(AllowanceTxtBox.Text)))
        Catch NumberProblem As Exception
            MessageBox.Show("There was a problem with your numbers for the sales price.", "Data error", MessageBoxButtons.OK, MessageBoxIcon.Error)
            SalesPriceTxtBox.Focus()
            SalesPriceTxtBox.SelectAll()
        End Try