When Is Thoughtfulness Morally Necessary?

Ethicists use the term "supererogatory" for acts that are above and beyond the call of duty, that is, acts that are good, but not morally required. I've been thinking lately about how we draw this line.

You're standing in line at the grocery store with a bunch of items and the person behind you has only one thing. The nice thing is to let this person move in front of you in line. It will only inconvenience you slightly, whereas waiting for you to finish would take quite a bit longer. A thoughtful gesture. But is it necessary? If you are the one with the one item and you know that the person in front of you with the basket full knows full well that you would be through in a flash and refuses to be kind, do you have reason to be upset, to think bad thoughts about this person's character?

At what point does nice turn into necessary?