Set up a checkbox to apply a percentage discount?

Hi Tuan,

I need to create a checkbox in my site's registration form for certain members that if checked, it will give them a 10% discount. I looked at your Payment Form tutorial and it seems to offer a possible method to multiply by .1 with a textfield, but I'm not sure how I would do that with a checkbox list. Is it possible? And what would the proper entries be to set it up? The default value would be No -- no change in price.

On a related note, is it possible to use the minus sign in the fee values field to subtract specific amounts from the fee when summed?

Thanks,
azurelink