My next mortgage payment says $6.01 is due. Nothing about a $25 fee on there. $5.99 in principal, $0.02 in interest.
I would bet money there is nothing on the agreement. I don't have the patience to go read it. That would be too much like work.
I think this is just a fee they charge and they have no basis for it except that when people go to pay off their mortgage, they can charge it.
We shall see. It's gonna be fun!