Answer

I have heard several halakhic decisors who were lenient about this. Though at first glance the reason is not clear, since this would seem to be payment for waiting for the money in the hands of the service provider. He gave you more delayed merchandise in exchange for your money. I now noticed an apparent contradiction on this in the Shulchan Arukh, Yoreh De’ah sec. 173, between סעיף 3 and סעיף 7, and I do not have time at the moment to get into the topic. See also Torat Ribit ch. 8, sec. 5, which brings a dispute on this.
I have now seen a responsum by a former student of mine, who writes that this is permitted in the case of classes: https://www.dintora.org/shut/234
In short, it seems to me that there is room to permit it.