In that you are now aware of their propensity to want to collect the late fees, I would send that mortgage payment in as soon as you get it in the mail. I know it's not fair and their going to have the advantage either way, but it's just not worth the time, effort, aggravation, to try to get those late fees adjusted.

Good for you staying on top of what's going on with your payments. Don't throw any of the statements away either. I don't know what government agency regulates the activities of this type of account, but it may be worthwhile to find out and then give them a call.