in the example letter it states  

"What has been happening, is that your corporation is cashing my personal checks/accepting my electronic funds transfers as payment while also redeeming the coupon which is double dipping and a crime! This is malfeasance and is trust fraud! ...

I am now aware of this illegal activity that your corporation is committing, but I am willing to overlook this crime, and make you an offer."

Would that be considered blackmail?  Could there be the possibility of that type of backlash?