Cannot get derived class to perfom function - vb.net

Question

I am working on an assignment where I have to create a base class named Bonus. The class should contain two Public properties: a String property named SalesId and a Double property named Sales. I also have to include a default constructor and a parameterized constructor in the class. Also include a GetBonus method (function) that calculates a salesperson’s bonus using the following formula:
sales* .05
Then create a derived class named PremiumBonus. The derived class’s GetBonu method should calculate the commission as follows:
sales * .05 + (sales – 2500) * .01.
Be sure to include a default constructor and a parameterized constructor in the derived class.
This is what I have so far but its only calculating the Bonus not the PremiumBonus when necessary.
Payroll.vb
Option Explicit On
Option Strict On
Option Infer Off
' base class
Public Class Bonus
'declare properties
Public Property SalesId As String
Public Property Sales As Double
'default constructor
Public Sub New()
_Sales = 0
_SalesId = String.Empty
End Sub
'parameterized constructor
Public Sub New(ByVal strId As String, ByVal dblB As Double)
_Sales = dblB
_SalesId = strId
End Sub
'method to calculate the bonus using formula
Public Overridable Function GetBonus() As Double
'returns the sales person's bonus
Return _Sales * 0.05
End Function
End Class
' derived class
Public Class PremiumBonus
Inherits Bonus
Public Sub New()
MyBase.New()
End Sub
Public Sub New(ByVal dblB As Double, ByVal strId As String)
MyBase.New(CStr(dblB), CDbl(strId))
End Sub
Public Overrides Function GetBonus() As Double
Return MyBase.GetBonus + (Sales - 2500) * 0.01
End Function
End Class
Main Form.vb
Option Explicit On
Option Strict On
Option Infer Off
Public Class frmMain
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click
Me.Close()
End Sub
Private Sub txtSales_Enter(sender As Object, e As EventArgs) Handles txtSales.Enter
txtSales.SelectAll()
End Sub
Private Sub txtSales_KeyPress(sender As Object, e As KeyPressEventArgs) Handles txtSales.KeyPress
If (e.KeyChar < "0" OrElse e.KeyChar > "9") AndAlso e.KeyChar <> "." AndAlso e.KeyChar <> ControlChars.Back Then
e.Handled = True
End If
End Sub
Private Sub txtSales_TextChanged(sender As Object, e As EventArgs) Handles txtSales.TextChanged
lblBonus.Text = String.Empty
End Sub
Private Sub btnCalc_Click(sender As Object, e As EventArgs) Handles btnCalc.Click
' calculates and displays a bonus
' if the sales are over $2500, instantiate a PremiumBonus object
' and then calculate the bonus
' otherwise, instantiate a Bonus object and then calculate the bonus
Dim myBonus As New Bonus
Dim myPremiumBonus As New PremiumBonus
Dim dblSales As Double
If dblSales > 2500 Then
Double.TryParse(txtSales.Text, myPremiumBonus.Sales)
dblSales = myPremiumBonus.GetBonus
Else
Double.TryParse(txtSales.Text, myBonus.Sales)
dblSales = myBonus.GetBonus
End If
' display the bonus
lblBonus.Text = "ID:" & (txtId.Text).PadRight(6) & dblSales.ToString("C2")
End Sub
End Class