That sounds like a good argument for Humble Bundle to have a convenient way for purchasers to go back and make a second donation after purchase.
They do, you can go and up your price after buying:
“Like what we're doing? Click here to increase your order amount!”
I'm pretty sure every Humble Bundle I've ever bought has had a "Like what we're doing? Click here to increase your order amount!" link on the download page, so they've already covered that base. :)