Need help with calculating the accumulated total

Hey everyone! Newbie here! :D
I've been stuck at a particular programming problem. I have to create an application for selling T-shirts. I have to display the items purchased and calculate the total. Supposed to be easy, right? Arghh!

So far I've got:
A ComboBox - this lists the shirts sizes (xsmall. small, medium, large, xlarge, xxlarge). (name: comboShirt)
Quantity Textbox - This is beside the ComboBox. This is where i enter the quantity of the shirts. (name: txtQuant)
OK Button - When the user clicks this button, the shirt item and quantity will appear in the listbox. The total amount will appear in the textbox below. (name: btnOK)
A ListBox - This is where the items selected by the user will appear. (name: lstOrder)
Amount TextBox - this is where the total amount will appear. (name: txtPay)

The prices of the shirt are: small, medium, large, xlarge = $16.00 each
and xsmall, xxlarge = $20.00 each

Here's a link to the screenshot: tinypic.com/r/w17dw4/7

Seems pretty basic. But I just cant come up with a code to calculate the total! Can anyone give me any idea as to how to go about that? Do i do a loop? Do an If else statement?
Also, does the total show up when i click the OK button? Or should i make another button, which the user will click after ordering to calculate the total? I'm totally lost.

This is the only thing i could come up with. But it doesn't really work. I'm not even sure how to add the $20 prices in this:

Code:

private void btnOK_Click(object sender, EventArgs e) { double total = 0, purchase=16, gtotal, quantity; quantity = double.Parse(txtQuant.Text); while (quantity != 0) { gtotal = quantity * purchase; total += gtotal; } txtPay.Text = purchase.ToString(); lstOrder.Items.Add(comboShirt.Text + "\t" + txtQuant.Text); }

I'd really appreciate any help with this! I have absolutely no clue how to go about calculating the total. Please point me in the right direction!

Thanks!