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Then, you could put a simple validation script to catch the onChange method of the box. If the value of the box <> 0 alert("Don't change the price, eh?")
I'm sure there are lots of ways to handle it. But I like active server pages.
The other part is that you'd need a cgi to do it right. When the form page is generated, there should be a if-then statement to see if there is the price. If it isn't, they get one version of the checkout page, and if there is, they get the other one.