That’s not altruism, if you want to do it it’s not a sacrifice. Unfortunately altruism lays claim to both positive things (benevolent actions chosen by an individual such as the situation you describe) and negative things (sacrificing yourself, i.e. doing something you don’t want to do through shame, guilt, ect.). We need to cut the good from the evil and reclaim it as the good. Altruism means to prioritise another over yourself, and it claims this is the moral ideal, that when you do not pursue it you are being evil. So the idea you can do it some of the time and not all of the time would not be accepted as moral by a genuine altruist. But the problem is, it’s clearly impossible to do it all of the time. It demands the impossible and damns you to an eternity in hell for not following it completely.

I say, why not do the good this all the time. Pursue your values always. If you value positive interactions with strangers, that’s perfectly valid and not altruistic sacrifice like they would lead you to believe. I don’t mean to be too critical, but we live in a world dominated by the idea that altruism is the good, there’s a lot of unpacking to do.