And in any circumstances, the ludicrous notion of adding a contract after a purchase is made is unconscionable.

You buy a car. You pay cash. It’s yours. Afterward, the seller says “oh, and you have to agree to these terms before you’re allowed to start it.” Proper response: fuck off, I already paid you. It’s the same here. If you want to apply extra terms and conditions, negotiate them before the conclusion of the purchase. Anything afterward should be utterly laughed out of court.