I have recently heard that saying kaddish without a fee has no benefit for the soul. If a stranger says kaddish then what did that person do in their life to create this person into someone who says kaddish? Rather, it seems that a fee has to be paid from the estate so that it is the deceased who is making this kaddish be said.

Shalom. There obviously is benefit for the soul when every kaddish is said, even without pay. What you heard is probably based upon the Beit Yosef (Y.D. 403) where it seems that for one who isn’t a relative, it is preferred that he be paid. The following explanations are suggested: a. As you said, there is that much more merit for the mitzvah of the deceased when he or his family have personally influenced or induced a mitzvah, even through pay. b. The “kaddish-sayer” will feel more of an obligation and will be that much more conscientious in saying it properly and with concentration (kavana). c. It is usually a relatively poor person who supports himself through such an “occupation”, and therefore, one adds the mitzvah of charity. In any event, it’s always better, if there are sons, for them to say the kaddish. In addition, if the family isn't religious, and they may not understand the aforementioned explanations, and maybe even resent the fee, this may cause a chilul Hashem (disgracing God’s Name) and it's better not to ask for a fee, for a chilul Hashem is one of the most severe transgressions, and surely disturbs the honor of the deceased. With Love of Israel, Rav Ari Shvat