https://github.com/whit537/www.gittip.com/issues/113
The short story is that Gittip is targeted at funders, and the current wisdom is that one-off payments would erode confidence in the stability of one's funding stream. Yes, you can do what you were planning to do, pay for a week and then cancel, but Gittip doesn't want to optimize for that, I don't think.
In general, getting the money into systems is the hard part. If money is there, people will come - easy! Getting people to give money - whether to a company or for donations - that's hard!
Also, frankly, this is a nice proposition from the POV of the "funders". You seem to have built it for yourself. But, the value for the donors? Sounds like a tough sell.
So making donors happy is most important. Without them you're toast. With them you can AT LEAST support yourself, if nothing else :)
[0] http://www.theatlantic.com/technology/archive/2012/07/the-po...
Long answer: You're right, and I want to expand beyond GitHub. Here's the ticket: