Allowing one free attribute

I am facing a problem. I have an attribute setup for leash colors, you may choose up to 4 colors. (i already wrote the javascript to limit only 4 checkboxs) but here it the problem I am running into. They want the first color to be free. example:

LEASH
-color attribute:

[] blue: $10.00
[X] red: $10.00
[] yellow: $10.00
[X] orange: $10.00
[X] purple: $10.00
[] black: $10.00
[] green: $10.00

the total attribute charges should now be $20.00 (not $30.00)

Any ideas? Much appreciated. Thanks

There isn't anything to build in a Free One and force a total of 4 on checkboxes ...

You could do dropdown sets where the first set is free and the other 3 cost $10 ... and force it so all 4 require 1 selection using the Default and Display only as one of the choices on each of the 4 sets ...

Hmm, that is an interesting approach to it. Thanks Linda.

I ended up just modifying the /classes/shopping_cart.php file to look for this One-time-charge, and if there are more than one selected it will ignore one of them. Wish there was an easier way to do that then modifying a class file but that seemed to do the trick.

If anyone wants a detailed description please let me know and I would be happy to post it here.