How to let buyers select custom price, allowing payment to go to different parties? (like Humble Bundle)

Looking at Humble Bundle as an awesome ecommerce example here, where a buyer can either select predetermined tiers (ex. $1, $8, $15, $25) for specific bundles, or can determine their own custom amount between or above those tiers.

Here’s an example:

Purchasers also have the option to determine how they want the total custom amount to be divided between company who created the product (Wizards of the Coast), a charitable organization (Extra Life), and the reseller (Humble Tip).

3 Questions:
-It seems like I could set up the pre-determined tiers easily enough, but i’m not sure how I’d manage the custom costs between or above the tiers.
-Is it possible to create sliders for dynamic cost selection?
-How can I connect purchasers to PayPal Giving Fund or allow purchases to partially go to a charitable org?

Thanks!