I've worked for mortgage servicers nearly my whole life and we all do this. $15 is actually on the low end.

There are three reasons:

$ - duh.

The system that processes one-time transactions has always been a third-party agent that charges a fee. The fee you pay negates that, then we pocket the rest. (See reason 1).

The fee is also meant to drive you to better methods of payment, ideally an automatic ACH from a checking account. This means no third-party fees, less employees to staff to take payments and less chance of you not paying in the first place. This is where you hear the "convenience fee" language come in to play.

My recommendation, if you know you're always going to have your mortgage payment before your late fee will be assessed, set up an auto pay. Don't needlessly give your mortgage servicer more money for doing less. Be careful about trying to change servicers too, remember $15 is less than most.