The only way that I have found to do something similar is to add a 'donation' product to the cart. In that case the amount 'rounded up' the total to the nearest dollar. Creating a variable price product is kind of interesting. I'll pm you the address of that site.

Another way of doing what you want might be to write an order total module that added the percentage. The user's selection would need to be stored in a session variable probably with the dropdown setting that session variable.

Basically, it is going to take a bit of coding and might be beyond the scope of a forum post :-) but should be possible.

Maybe someone else has a better solution.