I might just poke my nose in here, since one of my sites sells e-books.
I decided to have a play with the settings on my cart to see if I could reproduce that "invalid selection" error you were having.
If I set the attribute to "display only," then when I try and add that product to my cart, I get the "invalid selection" error. Obviously this is not a "display only" attribute, so it not be set as such. But perhaps this is your problem?
So go to
admin=>catalog=>attributes controller and choose any of your products.
You should see the option name "downloads" and beside it the option value "zip."
To the right you will see a row of icons, if you hover your mouse over them you will see what the icons are for. "display only", "free", "default" etc.
The "display only" setting should be false, so it should show with a big red cross through it. By clicking on the icon, you can toggle the value from true to false.
When you have done that, try putting the product in your cart and see if the error still shows.
Good luck