Adding together listbox values and outputting them to a textbox then adding that textbox together with another textbox?

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C#

So I am working on a small project and I have buttons lots of buttons. These buttons fill in a cost in a listbox and update a running total in a textbox below it. What I want to happen is the resulting textbox to be added with user values from another textbox when a radio button is selected. the code below is the button to add the product to the listbox and textbox and also the code for the radio button.

private void btn_product_Click(object sender, EventArgs e)
    {
        listBox1.Items.Add("01.25");
        Decimal total = 0;
        foreach (var v in listBox1.Items)
        {
            total += Convert.ToDecimal(v);
        }
        resultTextbox.Text = total.ToString();

    }


    private void rdCash_CheckedChanged(object sender, EventArgs e)
    {
        if (rdCash.Checked)
        {
            decimal total = Convert.ToDecimal(resultTextbox.Text);
            decimal paid = Convert.ToDecimal(txtPaid.Text);
            decimal results = Decimal.Add(total, paid);
            txtChange.Text = results.ToString();
        }



    }

Im getting an error at the decimal total = Convert.ToDecimal(resultTextbox.Text); line that says that it is not a correctly formatted value any help to solve the problem I am having would be appreciated and I hope it is just something I overlooked thank you!



Edit: I should mention that there is more to the if statement i just have not added it yet as I am stuck on this problem. Also Visual Studio shows no errors until I run to this line in the program.




IMAGES OF THE PROJECT AND ERROR:

http://imgur.com/50R001b[^]
http://imgur.com/ej6i2qN[^]




there is probably a more efficient way to accomplish this register than how I am doing it so please feel free to add input on what I could change to keep it simple as compared to what I have.