You've gotten the correct, technical responses you need and all I will add is that you need to verify that state law does not consider such a charge as interest and, therefore, usurious. My personal observation is, however, that this truly is a fee related to the time value of money, not a service. If I went to work for CFPB today, it would be my first target.
_________________________
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.