"Take the discount or donate?"

I just noticed a new (well, new to me) feature on Slate called "My Goodness" that gives advice on how to make the world better. It's pretty neat, actually, and the latest Q&A was particularly relevant to my life: what to do when supermarket cashiers ask you to donate to charity at checkout. I shop mostly at Whole Foods, and I have dutifully purchased several of those reusable green bags to do my little part for the environment. (Baby steps.) When I go to pay, the cashier asks me whether I want to take a bag deduction--5 or 10 cents per bag--or donate that money to whatever charity is on the menu that day. I almost always take the deduction, and feel just a little bit selfish when I do it. But I figure I'm currently the object of some form of charity myself (in the form of subsidized student loans, where the interest is paid by the good ol' taxpayer), and so long as I'm in the red, I can't really be expected to give to others. Then again, it's just a nickel or two on top of a $100 grocery bill, and not at the cheapest store in town, either. But on the other hand, I did pay for the reusable bags, so maybe I'm entitled to recoup my investment, and then some, before thinking about giving that discount away? And why should Whole Foods get to decide where my money goes, anyway?

Thoughts welcome, as always, in the comments.

Also, pardon the quality of the writing as I get back into the rhythm of this thing. My muscles are atrophied and need some TLC before they work properly again.