Not really - as that site stands it is a demonstration of it being added as a variable price product rather than an order total. So it happens at cart level rather than checkout level.

If you wanted it to happen at checkout rather than shopping cart stage, it would probably need to be an order total nodule that worked on a session variable. There'd need to be a bit of thought about that because you'd need to refresh the page if someone chose to donate and ensure that all the information was passed to the refreshed page.

It would however, be a better system because you could accommodate shipping and taxes. And because the other version is a bit of a hack rather than a beautiful module to be honest.

It's do-able but would involve entirely new coding. So, it probably depends on whether you can do that coding yourself or if you are happy to pay someone to do it for you.