Pricing based on multiple versions (attributes)?

I'm trying to figure out the correct combination of pricing attributes to use, but I can't quite get it right.

I sell software add-on utilities as downloadable products. I also offer different versions of these same utilities for different versions of the parent application.

My quandry is that, say a person owns versions 3 & 4 of the parent app and want to get my utilities for both versions - it's the same product, but two versions.

I want pricing to be that if they order version (x) it costs $5; if they want to also get version (y), it's $2 more. Or vice-versa - if they want version (y) only, it'$5 but if they get both it'll be a total of $7.

Right now I'm using checkboxes for customers to select the version they want.

Is there a way to determine that if 1 checkbox is selected, the cost is $5 but if two checkboxes are selected, the cost goes up to $7?

Any ideas? many thanks

Might get confusing but offer the product with version attributes as radio buttons that only allow one to be selected at the base price

Then make an add on attribute for additional versions with the plus $2.00 added

i.e. Product "X"
version a
version b

Add Additional version to above
version a
version b

thanks - I'm going to try that.

One other thing I though of, that might be a bit more user-friendly, is to create three downloads - one for version x in a .zip, one for version y in a zip, and a third (with the price difference) containing both in a single zip. Then the customer can select from a dropbox instead of the checkbox I'm using currently.

Duh. Sometimes the simple solution is the best solution!

Yeah that sound better anyway