Old thread, but I was looking for something similar, and felt I had to respond to Simon. Simon, I wanted to point out that many of us working for charities do not have lots of coding experience, nor the time to spend trying to develop such a modification, if Pablo is anything like me, I am here at 3:45am on a bank holiday weekend working because I have so much work to do during the week. So to be frank, your reply to pableset was not exactly very charitable was it?
But back on subject... if anyone does have such a contribution that prompts a shopper to add an additional donation at checkout then please do post, I and others would find it most valuable.
Pablo, for now I have done a crude workaround that places a small text link at checkout_confirmation.php asking if they would like to round up by making a donation to our charity, it then takes them to a product donation category, where I have set products in multiples of 0.01, 1.00. 10.00 and 50.00, allowing them to round up their purchase. Not as nice as having a box at checkout where they can enter an amount to donate and click re-calculate, but it will do until I can find something better. Hope that helps those still searching.