FWIW, and I obviously don’t know my way around the code, I modified the Load_coupon method for the Item class to check the coupon value and if >= product price then transform it into a 100% off, which is then handled properly by the rest of the plugin code.
if(intval($this->coupon[‘discount’])>=$this->get_price() &&
$this->coupon[‘discount_type’]===’fixed’){
// transform this to a 100%
$this->coupon[‘discount’]=100;
$this->coupon[‘discount_type’]=’perc’;
}