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A running total - you could be selling anything whereby the client will see immediately how much the bill is (excluding tax or shipping).
You'll need the javascript from here.

In the qty part of the form, you'll need an id for each product for the script to identify. The format is PROD_PRICE, for example id="PROD_9.00" none of these values are passed to the cart, only the normal qty, product and price plus any others you may have inbuilt in your form.