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 valid contract under US common law (which has it’s roots in English common law) requires three things: offer, acceptance, and consideration (which means each side gives up a legal right in exchange for something, i.e. you become bound to pay a monthly fee in exchange for the other side being obligated to provide a service). So when a company sends you a contract with pre-set terms (an offer), you can either sign that pre-drafted contract (acceptance), ignore it (rejection), or change one or more terms and send it back to the drafter to either accept or reject (rejection and counter-offer).
Every time language is changed it rejects the previous offer and creates a new counter offer. You don’t have a “contract” until an offer is accepted with no changes (also known as mutual assent).
As a practical matter, no large company is going to accept a counter-offer for a contract with one consumer. Their entire apparatus is constructed to serve the terms of a form contract. There is no practical way for a company with thousands/millions of customers to honor the unique terms of one customer’s contract.
With that being said, even if a company could theoretically honor the modified contract, they aren’t going to risk it. They would have to have a lawyer or legal employee review it and analyze its costs and benefits to the company. That would cost a lot of money per each modified contract, so the company will simply ignore the changes and move on the the next customer.
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/