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)?
I can’t find it, but I seem to recall back in the day this one absolute legend of a chanced got a contract from his bank. He wrote a new contract, which he carefully designed to be visually similar to the one he received, but which gave him incredibly favourable terms with massive penalty fees on the bank should they try to renege on it. Nobody at the bank noticed the signed contract wasn’t the one they had sent out, leaving them on the hook. And since they had freely agreed to it, a judge ruled it was valid and enforceable.
From another comment here https://detechter.com/man-creates-credit-card-sues-bank-for-not-paying/