multiple inputs in text box not totaling - vb.net

Question

I got another super basic question, im trying to total the subtotals of every entry in the txtPrice.Text the user enters, and then refresh the other lables with the updated tax, shipping, and grand total. Its not totaling the subTotal, everything else works fine. Whats up with that?
Private Sub btnCalc_Click(sender As Object, e As EventArgs) Handles btnCalc.Click
Dim sglSub As Single
Dim sglTotal As Single
Dim sglSalesTax As Single
Const TAX_RATE As Single = 0.02
Dim bytShippingCharge As SByte = 10
Dim sglCompTotal As Single
Single.TryParse(txtPrice.Text, sglSub)
sglTotal += sglSub
lblSubTotal.Text = sglTotal.ToString("C2")
sglSalesTax = (sglTotal * TAX_RATE)
lblTax.Text = sglSalesTax.ToString("C2")
If sglTotal >= 100 Then
bytShippingCharge = 0
End If
lblShipping.Text = bytShippingCharge.ToString("C2")
sglCompTotal = (sglTotal + sglSalesTax + bytShippingCharge)
lblTotal.Text = sglCompTotal.ToString("C2")
End Sub

Answer 1

Tips
In this line:
sglTotal += sglSub
-Every time you work with a total initialize it to zero before adding a value to it. If not it can leads to undesired result.
-When working with currency is better to use a decimal type instead.
If you want a variable keeps its value declare it shared.
This a little example of how you can use a shared field
Public Class Form1
Shared total As Decimal = 0D
Shared Sub calc()
total += 2
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
calc()
Label1.Text = total.ToString
End Sub
End Class