My parents got duped by this on their mortgage, as well, for years. If you send extra, you have to note in giant clear letters 'APPLY TO PRINCIPLE'. If you pay by check, I would send two separate checks --one for reg pmt due, and a separate one for that $50 with the "For" section of the check notated with the apply to principle.

Either that, or call and make the extra payment with the CSR applying it to principle. It's a stinky trick.