How to add value on button click C# winform

newbie question. I want my winform button to add the value on each click in a total textbox. for example if each click is 4.25 then 2 clicks would be 8.50. any information would be great.

private void BtnLarge_Click(object sender, EventArgs e)

    {
        float largeC = 4.25F;
        TxbInvoice.Text += "Large Coffee......" + largeC + Environment.NewLine;
        txbtotal.Text += largeC++;

    }