I am selling books in a store.
For every product I make the book have a price for $0.
I then set up radio attributes for edition
example
Print Version $50
Web Version $40
PDA $40

etc
So that I do not need to make multiple listings for each product

I then always make Print version Default Attribute
to be Marked Selected

Which works out ok, so that print will always be checked.

However, I use firebug (where you can make updates to html pages on the fly to see how the layout changes) and I can then make it so Print Version is unchecked. Then click add to cart, and a price of 0 shows up. So clearly there is no validation because you can never trust the client machine.
Am i just doing something wrong? Or am I going to have to code my own validation script?

I mean 99.9% of people won't bother cheating the system like this, and if they did I would obviously not honor the order. But it just seems kind of lame that with a simple 'hack' I can put in an order for a free book because I changed a bit of HTML on the client