You could defer all the "no" dollars into a pile so the user could add a bonus dollar in the same way for another day.
Edit: I think you added this to your reply already! I love the idea, though.
To solve this exact problem our model is a bit different. Our daily email features one cause and outlines two charities addressing the problem in a different way.
(Ex Cause: breast cancer Charity 1: Research Charity 2: Support for survivors. You can choose to donate to either of those or if you don't like the cause of the day you can rollover your donation to the next day.
Rollover as much as you like :) We have a few users rolling over 10 days in a row until they hit a cause that resonates with them.
On one hand I agree that it would be nice to opt out of charities you disagree with. However I think in doing so it would destroy "Dollar a Day's" USP.
To me the USP of "Dollar a Day" is: Completely hassle free donating at an affordable cost ($30/month) which gives you almost endless positive feedback (i.e. every day you're reminding of why you're a "good person").
As soon as you start adding complexity then the USP changes from "completely hassle free" to "something I have to check every single day." Plus people will have to grasp how the whole skip system works (e.g. if I skip a day, is it still $30/month? If it is then where does that $1 go? What if I skip tons of days? What if I don't redistribute that $1 by the end of the month, where does it go, etc).
So I agree with your point, but I disagree that "Dollar a Day" should implement it. Instead they should stick to their USP and make very sure they don't mis-fire on the charity selection front (e.g. "Today is a $1 donation to the Republican Party," "Today is a $1 donation to Susan G. Komen," etc).
A useful feature might be the ability to opt out by category or keyword. Maybe I'm not interested in animal charities, or maybe abortion is against my religion, or maybe I don't trust any charity with the word "global" in its name for some petty reason. Whatever it is, I need to be able to opt in to funding the 29 charities I'm cool with and opt out of the one I don't like.
However, I _hate_ the idea of donating to a black box. True, Dollar a Day publishes its criteria for selecting nonprofits, but those criteria are highly subjective. And, according to the FAQ, there's no way to opt out of donating to a particular featured nonprofit that you don't want to support. Heck, even if you cancel your monthly "subscription," your money will still go to all of the nonprofits that are in the queue for the rest of the month.
I understand the organization's decision to make things simple to start out, but I hope that it soon offers a way to opt out on a charity-by-charity basis (it could be as simple as a link in the daily email), and pro-rated refunds so that if you decide to stop donating on Day 1 of the month, you're not on the hook for days 2-30.