It’s common to receive a mail from some service that they’re changing the terms and conditions of their service (internet provider, gmail, etc.) These changes are unilateral and can be about anything, from their privacy policy to the money they intend to charge me.
Why can’t I unilaterally send them a mail and say, I am changing the terms of my patronage and will now insert my new policy (pay only xx amount, or demand a mint Chico chip ice cream every Tuesday)?
A significant counter example is click wrap. You can’t see the terms before you buy and you can’t reject it and get your money back.
Every instance of trying to set terms after purchase should be unenforceable in any just society