One thing that I've come with when thinking about personal budgeting, of all things, is the concept of granularity. For someone who is poor, the situation is analogous to yours. The dad, lets say, of the household might be having a similar attack of conscience as you are on whether he should buy a candy bar at the gas station, when there are bills that can't be paid.

But it turns out that a small enough purchase, such as a really cheap candy bar (for the sake of argument), doesn't actually make any difference. No bill is going to go from being unpaid to paid because that candy was bought rather than unbought.

So relax. Buy a candy bar every once in a while. It won't make a difference.