My problem is this is a private company, blatantly charging me out the ass, when I don't even know if they actually have the authority to do so.
Here in Ontario, Canada. They do not have that authority unless the person issuing it is sworn as a Municipal Law Enforcement Officer. This can be obtained by licensed security officers working for an agency which has obtained the required permits, and the security officer must also take training from the Police service(s) which has usual jurisdiction over the area(s) they are authorized tag vehicles in. They are issued a ticket book/machine by the police agency and the tickets themselves appear as identical to the ones issued by Officers employed the City.
Anyone can stick any sort of paper they want on your windshield, but if that paper implies it's a Parking infraction notice in the usual sense, it could be seen as impersonate peace officer. As in order to certify a notice of offence and lay a charge, at least here, one must be sworn as a peace officer for the purposes of the specific act the offence is found in, or else as a Police Officer for the purposes of Sect 2 of the Criminal Code (Federal Police agencies) or The provinces Police Services Act (Provincial and municipal police).
The thing that makes these private tags questionable, IMO, is that a tag issued by Police Officer/Parking Officer/MLEO sworn security officer gives you the right to bring that charge before a Court of competent jurisdiction, and dispute the charge in front of a Judge/JP and call the issuing Officer to the stand and question them and examine the evidence they have of the offence, as well as take the stand in your own defence. The judge can, and frequently does issue a finding of not guilty for a variety of reasons. Even when a finding of guilty is entered, the judge frequently will reduce the fine amount and/or give you an extended period to pay.
You don't get this with these kind of things. It's completely arbitrary, with no chance to defend yourself or have it examined by a disinterested 3rd party. You either have to pay, or have your credit fucked up.
That is, to me, not just.