Attribute discounting question

Hey Zenners,

I've been searching all day for an answer to this on the forum with no luck.

I have 3 downloadable product attributes at the same price with checkboxes so they can select more than one of they want.
(one is checked by default for the customer)

Can Zen-Cart automatically discount a 2nd and/or the 3rd choice either by price or percentage?

I don't want to worry which of the 3 attribute choices the customer chooses first. I just want the first attribute choice to be the full price as marked, and then any other choice to be discounted.

Thanks everyone for the help in advance

===============
Please choose from one or more file formats:

mp3+G [karaoke] ( $5.00 ) - SELECTED
mp4 video [iPod/iPhone] ( $5.00 )
wmv video [PCs/Zune] ( $5.00 )

Plus I forgot....
If I make the price on the item $0.00 and the customer unchecks all the checkboxes and tries to Add To Cart, I was hoping to get an error message to select an attribute, instead of adding a $0.00 item to the cart that does not have a download link attached to it anyway.